MXS-2141: Retry query on master if it times out on slave

With causal_reads enabled, the query would return with an error if the
slave was not able to catch up to the master fast enough. By automatically
retrying the query on the master, we're guaranteed that a valid result is
always returned to the client.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -73,6 +73,7 @@ public:
     {
         NONE,
         WAITING_FOR_HEADER,
+        RETRYING_ON_MASTER,
         UPDATING_PACKETS
     };