Make execute_session_command virtual

Making the function allows higher level checking to be done by the derived
class.

The readwritesplit does some of the reply bookkeeping for session commands
in the function. This makes their execution less prone for errors as the
states are always updated correctly whenever a session command is
executed.

server/modules/routing/readwritesplit/readwritesplit.hh

@@ -178,6 +178,18 @@ public:
         m_reply_state = state;
     }
 
+    bool execute_session_command()
+    {
+        bool rval = mxs::Backend::execute_session_command();
+
+        if (rval)
+        {
+            set_reply_state(REPLY_STATE_START);
+        }
+
+        return rval;
+    }
+
 private:
     reply_state_t m_reply_state;
 };

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -263,7 +263,6 @@ bool route_session_write(ROUTER_CLIENT_SES *rses, GWBUF *querybuf, uint8_t comma
 
                 if (expecting_response)
                 {
-                    bref->set_reply_state(REPLY_STATE_START);
                     rses->expected_responses++;
                 }