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.

dbsim/db_server_state.hpp

@@ -45,7 +45,6 @@ namespace db
                 "",
                 address,
                 working_dir,
-                wsrep::gtid::undefined(),
                 1,
                 wsrep::server_state::rm_async)
             , mutex_()