CBL-13:  kill child processes using bash EXIT trap

Change-Id: I6e348be9bdd42d8c0356f9a3e101dc0c8c6ce9e8
Reviewed-on: http://review.couchbase.org/37687
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Philip Labee <philip@couchbase.com>