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.

include/maxscale/protocol/mysql.h

@@ -547,4 +547,13 @@ bool mxs_mysql_extract_ps_response(GWBUF* buffer, MXS_PS_RESPONSE* out);
  */
 uint32_t mxs_mysql_extract_ps_id(GWBUF* buffer);
 
+/**
+ * @brief Determine if a packet contains a one way message
+ *
+ * @param cmd Command to inspect
+ *
+ * @return True if a response is expected from the server
+ */
+bool mxs_mysql_command_will_respond(uint8_t cmd);
+
 MXS_END_DECLS