Pass initial position when loading the provider

Having initial position as a part of server_state does not allow
restarting the provider from different startup position without
either reconstructing server_state or adding setter method for
initial position. As the only use for initial_position in server state
was to pass it to provider during provider load, it makes more sense
to have the initial position to be passed as an argument to
server_state load_provider() method.

test/mock_server_state.hpp

@@ -214,7 +214,6 @@ namespace wsrep
                           wsrep::server_service& server_service)
             : wsrep::server_state(mutex_, cond_, server_service,
                                   name, "", "", "./",
-                                  wsrep::gtid::undefined(),
                                   1,
                                   rollback_mode)
             , mutex_()