Rename go version dependent sources using idiomatic naming

[dolmen]

Description

The Go toolchain has special support for build tags for Go versions in source filenames.
This makes build tags redundant, so they are removed.

Checklist

• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing
• Extended the README / documentation, if necessary
• Added myself / the copyright holder to the AUTHORS file

[methane]

Can we drop Go 1.7 support in next release?

[julienschmidt]

Can we drop Go 1.7 support in next release?

yes

[shogo82148]

The Go toolchain has special support for build tags for Go versions in source filenames.

The job https://travis-ci.org/go-sql-driver/mysql/jobs/392633176 fails.

https://golang.org/pkg/go/build/#hdr-Build_Constraints
If a file's name, after stripping the extension and a possible _test suffix, matches any of the following patterns:
*_GOOS
*_GOARCH
*_GOOS_GOARCH

As you have said that Go toolchain has special support in source filename, but *_GOVERSION is not in the patterns list.
So, // +build go1.8 is required, isn't it?

[dolmen]

It appears I had an incorrect understanding of the build documentation: there is no magic for go versions in filenames.