Refactor readwritesplit session creation

Moved the initialization of the router session's member variables into the
constructor. Changed two functions that calculated server counts into the
router instance as they don't relate to a particular session.

server/modules/routing/readwritesplit/rwsplitsession.hh

@@ -73,7 +73,7 @@ class RWSplitSession
     RWSplitSession& operator=(const RWSplitSession&);
 
 public:
-    RWSplitSession(const Config& config);
+    RWSplitSession(RWSplit* instance, MXS_SESSION* session);
 
     // TODO: Make member variables private
     skygw_chk_t             rses_chk_top;