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/readwritesplit.hh

@@ -264,7 +264,7 @@ public:
     const Config& config() const;
     Stats&        stats();
     const Stats&  stats() const;
-    ServerStats&  server_stats(SERVER* server);
+    SrvStatMap&   local_server_stats();
     SrvStatMap    all_server_stats() const;
 
     int  max_slave_count() const;