MXS-1549: Prevent optimistic transaction with no slaves

If the router session does not have an already open slave connection,
optimistic transactions are not attempted.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -204,6 +204,9 @@ private:
 
     void handle_trx_replay();
 
+    // Do we have at least one open slave connection
+    bool have_connected_slaves() const;
+
     /**
      * Start the replaying of the latest transaction
      *