[async] Allow monitoring extra processes.
[async] Allow monitoring extra processes.

By default an async would monitor the process that started it and will
terminate if it dies. Sometime it's useful to "link" an async to some
extra processes. That's what this option is for.

Change-Id: Ie928c0a34f7d842d1f665632c0faa23a85f8fd18
Reviewed-on: http://review.couchbase.org/88110
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Tested-by: Aliaksey Artamonau <ce14b1c16e0fd654f926118eb8addbec5aff4ffa@couchbase.com>
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>

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