MXS-1632: Add per-server read-write statistics

The read-write distribution in readwritesplit is now stored in a map
partitioned by the servers that the router has used. Currently, the
statistics for removed servers aren't dropped so some filtering still
needs to be added.

include/maxscale/routingworker.hh

@@ -490,6 +490,12 @@ public:
         return get_local_value();
     }
 
+    // Dereference operator
+    T& operator*()
+    {
+        return *get_local_value();
+    }
+
     /**
      * Assign a value
      *