Split pilosa driver out of go-mysql-server

[smola]

It would be good to separate the pilosa implementation to a separate library, out of go-mysql-server, in order to keep go-mysql-server as the core, free of specific implementations as much as possible.

cc @mcuadros

[kuba--]

I believe we should have more generic package like go-mysql-index which you can import like you import go's std. lib. db package and driver which will anonymously implement the Index interface, e.g.:

import (
	"github.com/src-d/go-mysql-index"
	_ "github.com/src-d/go-mysql-index/driver/pilosa"
)

[erizocosmico]

Do we still want to do this?

[ajnavarro]

We can leave this to see if it makes sense to do it in the future. Right now is not causing any problems.