Add exit message handler for native index builder

Change-Id: If60d7eeef715d72caa89ccfbd3caac0daf28af59
Reviewed-on: http://review.couchbase.org/32545
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: buildbot <build@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>