MXS-2464: Retry failed session commands

If the execution of a session command fails on a master, it is retried
again. If the master is not available, the response will be returned from
one of the slaves.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -225,6 +225,7 @@ private:
     int              get_max_replication_lag();
     mxs::SRWBackend& get_backend_from_dcb(DCB* dcb);
 
+    bool retry_master_query(mxs::SRWBackend& backend);
     void handle_error_reply_client(DCB* backend_dcb, GWBUF* errmsg);
     bool handle_error_new_connection(DCB* backend_dcb, GWBUF* errmsg);
     void manage_transactions(mxs::SRWBackend& backend, GWBUF* writebuf);