MXS-852: Handle one-way session commands

Session commands that will not return a response can be completed
immediately. This requires some special code in the readwritesplit Backend
class implementation as well as a small addition to the Backend class
itself.

Since not all commands expect a response from the server, the queued query
routing function needs some adjustment. The routing of queued queries
should be attempted until a command which expects a response is found or
the queue is empty.

By properly handling these types of session commands, the router can
enable the execution of COM_STMT_CLOSE and COM_STMT_RESET on all
servers. This will prevent resource leakages in the server and allow
proper handling of COM_STMT type command.

server/modules/routing/readwritesplit/rwsplit_mysql.cc

@@ -104,27 +104,6 @@ is_packet_a_query(int packet_type)
     return (packet_type == MYSQL_COM_QUERY);
 }
 
-/*
- * This looks MySQL specific
- */
-/**
- * @brief Determine if a packet contains a one way message
- *
- * Packet type tells us this, but in a DB specific way. This function is
- * provided so that code that is not DB specific can find out whether a packet
- * contains a one way messsage. Clearly, to be effective different functions must be
- * called for different DB types.
- *
- * @param packet_type   Type of packet (integer)
- * @return bool indicating whether packet contains a one way message
- */
-bool command_will_respond(uint8_t packet_type)
-{
-    return packet_type != MYSQL_COM_STMT_SEND_LONG_DATA &&
-           packet_type != MYSQL_COM_QUIT &&
-           packet_type != MYSQL_COM_STMT_CLOSE;
-}
-
 /*
  * This one is problematic because it is MySQL specific, but also router
  * specific.