Can't create more than max_prepared_stmt_count statements

The following error is reported if enough prepared statements are created by a single connection:

Can't create more than max_prepared_stmt_count statements

The reason is that prepared statements aren't being closed correctly.

All prepared statement protocol commands receive their classification based on the COM_STMT_PREPARE when only COM_STMT_EXECUTE should receive its classification based on it. The end result is that the COM_STMT_CLOSE is routed to only one backend and it's left open on all others.