[async] Drop the exit message from async child.
[async] Drop the exit message from async child.

After producing result, the child immediately terminates. So the
parent would receive the exit message (because of my last minute
change) and terminate. Which is not what we want.

Change-Id: I21e41103d06380be0c6cc4a997d0dd3daa8ac633
Reviewed-on: http://review.couchbase.org/74640
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

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