[async] Return executor pid from register_child_async call.
[async] Return executor pid from register_child_async call.

That relieves the caller from the duty to pass both the controller and
executor pids.

Change-Id: I42a24270dc9b0af2f7fee703645c60e5e38daff1
Reviewed-on: http://review.couchbase.org/88111
Tested-by: Aliaksey Artamonau <ce14b1c16e0fd654f926118eb8addbec5aff4ffa@couchbase.com>
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server,membase/ns_server