Make "_" the default schema delimiter for SQLite and MySQL

Currently, "." is default schema delimiter for SQLite and MySQL. By definition a dot is allowed[1] as long as it is quoted properly but because `QueryGenerator.quoteIdentifier(s)`splits by dots this will fail hard.

To avoid this an error is thrown, but this will also break BC. However, I think this is ok in this case.

[1] http://dev.mysql.com/doc/refman/5.1/en/identifiers.html