Deprecate mysql-forward_autocommit

[renecannao]

mysql-forward_autocommit is incorrectly used (it creates complex and incorrect logics), while often users really need mysql-enforce_autocommit_on_reads.
It is also possible that the current implementation is broken, so deprecating it now makes sense.

We should:

• remove any reference in the code related to mysql_thread___forward_autocommit
• if mysql-forward_autocommit is set to true , report a warning in the error log saying that the variable is deprecated

This to be applied only to 2.1