COM_STMT_PREPARE halts the processing pipeline

The JDBC connector can send a COM_STMT_PREPARE command followed immediately by a COM_STMT_EXECUTE command. Currently MaxScale halts the processing of incoming packets while the COM_STMT_PREPARE is being executed (it's a session command) and only continues with the COM_STMT_EXECUTE once it's done.

There's no need to do that as all of the information needed to route is known at the time the COM_STMT_PREPARE is read. A COM_STMT_EXECUTE should be an exception and should be routed even if a session command is being executed.