_g_dbus_worker_flush_sync: always flush if we need to

_g_dbus_worker_flush_sync: always flush if we need to

We didn't previously flush in a couple of cases where we should have
done:

* a write is running when flush is called: we should flush after it
  finishes

* writes have been made since the last flush, but none are pending or
  running right now: we should flush the underlying transport straight
  away

Bug: https://bugzilla.gnome.org/show_bug.cgi?id=662395
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Cosimo Alfarano <cosimo.alfarano@collabora.co.uk>