Cache the local server statistics object

By storing the server statistics object in side the session, the lookup
involved in getting a worker-local value is avoided. Since the lookup is
done multiple times for a single query, it is beneficial to store it in
the session.

As the worker-local value is never deleted, it is safe to store a
reference to it in the session. It is also never updated concurrently so
no atomic operations are necessary.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -169,6 +169,10 @@ public:
 
     otrx_state m_otrx_state = OTRX_INACTIVE;    /**< Optimistic trx state*/
 
+    SrvStatMap& m_server_stats;     /**< The server stats local to this thread, cached in the session object.
+                                     * This avoids the lookup involved in getting the worker-local value from
+                                     * the worker's container.*/
+
 private:
     RWSplitSession(RWSplit* instance,
                    MXS_SESSION* session,