Read recovered position from sst_received() after initialization

In general the position where the storage recovers after a SST
cannot be known untile the recovery process is over. This in turn
means that the position cannot be known when the server_state
sst_received() method is called. Worked around the problem by
introducing get_position() method into server service which
can be used to get the position from stable storage after SST
has completed and the state has been recovered.

include/wsrep/server_service.hpp

@@ -141,6 +141,16 @@ namespace wsrep
             wsrep::client_service& client_service,
             const wsrep::id& own_id) = 0;
 
+        /**
+         * Get the current replication position from the server
+         * storage.
+         *
+         * @param client_service Reference to client_service
+         *
+         * @return Current position GTID.
+         */
+        virtual wsrep::gtid get_position(
+            wsrep::client_service& client_service) = 0;
         /**
          * Log a state change event.
          *