Add support for pipelined execution of COM_STMT_PREPARE

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.

The benefits of implementing support for this would be lower latency for any connector that uses the "immediate execution" style (e.g. JDBC and ODBC).