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/protocol/MySQL/mysql_common.c

@@ -1649,3 +1649,10 @@ uint32_t mxs_mysql_extract_ps_id(GWBUF* buffer)
 
     return rval;
 }
+
+bool mxs_mysql_command_will_respond(uint8_t cmd)
+{
+    return cmd != MYSQL_COM_STMT_SEND_LONG_DATA &&
+           cmd != MYSQL_COM_QUIT &&
+           cmd != MYSQL_COM_STMT_CLOSE;
+}