GDBusWorker: move flush async op into continue_writing()

GDBusWorker: move flush async op into continue_writing()

This makes it easier to schedule a flush, by putting it on the same code
path as writing and closing.

Also change message_written to expect the lock to be held, since all
that's left in that function either wants to hold the lock or doesn't
care, and it's silly to release the lock immediately before calling
message_written, which just takes it again.

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>