s3/notifyd: add async send/recv functions

s3/notifyd: add async send/recv functions

Previously, without this patch, if notifyd died for whatever reason, it
would be restarted from smbd. However, if its initialization failed and
it exitted again, there would be no child entry in smbd for it and it
wouldn't be attempted to restart it again.

This patch adds async send/recv methods for starting cleanupd that will
reschedule restart attempt every second in case initilisation failed.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12022

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>