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.

include/maxscale/session_stats.hh

@@ -15,7 +15,7 @@
 #include <maxscale/ccdefs.hh>
 
 
-#include <map>
+#include <unordered_map>
 
 #include <maxscale/server.h>
 #include <maxbase/average.hh>
@@ -59,5 +59,5 @@ private:
     maxbase::CumulativeAverage m_num_ave_session_selects;
 };
 
-using SrvStatMap = std::map<SERVER*, ServerStats>;
+using SrvStatMap = std::unordered_map<SERVER*, ServerStats>;
 }