analyzer: Check rule that try merge compatible indexes.

[ajnavarro]

We need to check the actual rule that makes use of SetOperations and Mergeable Lookup interfaces.

This rule will be in charge of checking all the obtained indexes and try to merge them all together.

If not all the indexes are mergeable, use the most merged Index Lookup and discard the rest.

[erizocosmico]

That's already implemented https://github.com/src-d/go-mysql-server/blob/master/sql/analyzer/rules.go#L1099-L1258

[ajnavarro]

Changed the title to test that implementation with all the actual new development.

[erizocosmico]

Depends on #212

[erizocosmico]

Blocked until #212 is done

[erizocosmico]

This is not blocked anymore