MXS-1506: Retry interrupted writes

If a query is interrupted that was sent to the master, it is now
retried. This allows all autocommit queries to be transparently retried if
the server in question fails.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -153,6 +153,7 @@ private:
     bool handle_got_target(GWBUF* querybuf, mxs::SRWBackend& target, bool store);
     void handle_connection_keepalive(mxs::SRWBackend& target);
     bool prepare_target(mxs::SRWBackend& target, route_target_t route_target);
+    void retry_query(GWBUF* querybuf);
 
     bool should_replace_master(mxs::SRWBackend& target);
     void replace_master(mxs::SRWBackend& target);