Races detected on running gitbase during the test

[lwsanty]

General relates to src-d/gitbase#955 (comment)

Pre-conditions

1. gitbase binary, built with -race, running
2. --parallelism= >= 2

Steps to reproduce
option 1 run test from src-d/gitbase#955
option 2 run query affecting UAST utils

Observed

==================
WARNING: DATA RACE
Write at 0x00c0000d7198 by goroutine 21:
  github.com/src-d/go-mysql-server/sql/plan.(*exchangeRowIter).Close()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/exchange.go:269 +0x105
  github.com/src-d/go-mysql-server/sql/plan.(*limitIter).Close()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/limit.go:77 +0x55
  github.com/src-d/go-mysql-server/sql.(*spanIter).Close()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/session.go:416 +0x71
  github.com/src-d/go-mysql-server/sql/plan.(*trackedRowIter).Close()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/process.go:142 +0x73
  github.com/src-d/go-mysql-server/server.(*Handler).ComQuery()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/server/handler.go:280 +0xf15
  vitess.io/vitess/go/mysql.(*Conn).execQuery()
      /home/lwsanty/goproj/gopath/pkg/mod/vitess.io/vitess@v3.0.0-rc.3.0.20190602171040-12bfde34629c+incompatible/go/mysql/conn.go:819 +0x20b
  vitess.io/vitess/go/mysql.(*Conn).handleNextCommand()
      /home/lwsanty/goproj/gopath/pkg/mod/vitess.io/vitess@v3.0.0-rc.3.0.20190602171040-12bfde34629c+incompatible/go/mysql/conn.go:749 +0x1705
  vitess.io/vitess/go/mysql.(*Listener).handle()
      /home/lwsanty/goproj/gopath/pkg/mod/vitess.io/vitess@v3.0.0-rc.3.0.20190602171040-12bfde34629c+incompatible/go/mysql/server.go:441 +0x13f8

Previous read at 0x00c0000d7198 by goroutine 27:
  github.com/src-d/go-mysql-server/sql/plan.(*exchangeRowIter).iterPartitions()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/exchange.go:182 +0x106

Goroutine 21 (running) created at:
  vitess.io/vitess/go/mysql.(*Listener).Accept()
      /home/lwsanty/goproj/gopath/pkg/mod/vitess.io/vitess@v3.0.0-rc.3.0.20190602171040-12bfde34629c+incompatible/go/mysql/server.go:243 +0x18a
  github.com/src-d/gitbase/cmd/gitbase/command.(*Server).Execute()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/server/server.go:79 +0xe43
  github.com/jessevdk/go-flags.(*Parser).ParseArgs()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/jessevdk/go-flags@v1.4.0/parser.go:316 +0x110a
  main.main()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/jessevdk/go-flags@v1.4.0/parser.go:186 +0x8a6

Goroutine 27 (finished) created at:
  github.com/src-d/go-mysql-server/sql/plan.(*exchangeRowIter).start()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/exchange.go:135 +0xa0
==================
==================
WARNING: DATA RACE
Read at 0x00c000d34d40 by goroutine 21:
  github.com/src-d/go-borges/libraries.(*mergedRepoIter).Close()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-borges@v0.1.3/libraries/iterator.go:57 +0x3e
  github.com/src-d/gitbase.(*repositoryPartitionIter).Close()
      /home/lwsanty/goproj/lwsanty/gitbase/partition.go:96 +0x7b
  github.com/src-d/go-mysql-server/sql/plan.(*exchangeRowIter).Close()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/exchange.go:273 +0xcb
  github.com/src-d/go-mysql-server/sql/plan.(*limitIter).Close()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/limit.go:77 +0x55
  github.com/src-d/go-mysql-server/sql.(*spanIter).Close()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/session.go:416 +0x71
  github.com/src-d/go-mysql-server/sql/plan.(*trackedRowIter).Close()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/process.go:142 +0x73
  github.com/src-d/go-mysql-server/server.(*Handler).ComQuery()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/server/handler.go:280 +0xf15
  vitess.io/vitess/go/mysql.(*Conn).execQuery()
      /home/lwsanty/goproj/gopath/pkg/mod/vitess.io/vitess@v3.0.0-rc.3.0.20190602171040-12bfde34629c+incompatible/go/mysql/conn.go:819 +0x20b
  vitess.io/vitess/go/mysql.(*Conn).handleNextCommand()
      /home/lwsanty/goproj/gopath/pkg/mod/vitess.io/vitess@v3.0.0-rc.3.0.20190602171040-12bfde34629c+incompatible/go/mysql/conn.go:749 +0x1705
  vitess.io/vitess/go/mysql.(*Listener).handle()
      /home/lwsanty/goproj/gopath/pkg/mod/vitess.io/vitess@v3.0.0-rc.3.0.20190602171040-12bfde34629c+incompatible/go/mysql/server.go:441 +0x13f8

Previous write at 0x00c000d34d40 by goroutine 27:
  github.com/src-d/go-borges/libraries.(*mergedRepoIter).Next()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-borges@v0.1.3/libraries/iterator.go:37 +0x1ae
  github.com/src-d/gitbase.(*repositoryPartitionIter).Next()
      /home/lwsanty/goproj/lwsanty/gitbase/partition.go:79 +0x61
  github.com/src-d/go-mysql-server/sql/plan.(*exchangeRowIter).iterPartitions()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/exchange.go:187 +0x26f

Goroutine 21 (running) created at:
  vitess.io/vitess/go/mysql.(*Listener).Accept()
      /home/lwsanty/goproj/gopath/pkg/mod/vitess.io/vitess@v3.0.0-rc.3.0.20190602171040-12bfde34629c+incompatible/go/mysql/server.go:243 +0x18a
  github.com/src-d/gitbase/cmd/gitbase/command.(*Server).Execute()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/server/server.go:79 +0xe43
  github.com/jessevdk/go-flags.(*Parser).ParseArgs()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/jessevdk/go-flags@v1.4.0/parser.go:316 +0x110a
  main.main()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/jessevdk/go-flags@v1.4.0/parser.go:186 +0x8a6

Goroutine 27 (finished) created at:
  github.com/src-d/go-mysql-server/sql/plan.(*exchangeRowIter).start()
      /home/lwsanty/goproj/gopath/pkg/mod/github.com/src-d/go-mysql-server@v0.4.1-0.20190821121850-0e0249cf7bc0/sql/plan/exchange.go:135 +0xa0
==================