[async] Make async's suitable for side-effect only computations.
[async] Make async's suitable for side-effect only computations.

Via perform/{1,2} functions. The advantage over simple spawn is that
the spawned process will terminate itself if its parent dies.

Change-Id: I87c91dacbf23556ed7911ac583828c24eed0ec75
Reviewed-on: http://review.couchbase.org/84705
Reviewed-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Well-Formed: Build Bot <80754af91bfb6d1073585b046fe0a474ce868509@couchbase.com>
Tested-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>

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