sqlparser: handle case sensitivity

https://dev.mysql.com/doc/refman/5.1/en/identifier-case-sensitivity.html
Case sensitivity depends on the context: columns, indexes
and aliases are not case sensitive, but table names and table
aliases are.
Merge branch 'master' into vtocc
Merge branch 'master' into vtocc
Merge branch 'vtocc'
Merge branch 'master' of github.com:youtube/vitess