CCBC-453: Add pause-at-end option to pillowfight

Adds a new --pause-at-end (-E) option to pillowfight. When specified,
pillowfight will pause after completing its workload (keeping
connections open) until it recieves user input.

Change-Id: I6cdff66a29150de0c24352f7eb11af62f2f2db7f
Reviewed-on: http://review.couchbase.org/38473
Tested-by: Dave Rigby <daver@couchbase.com>
Reviewed-by: Mark Nunberg <mnunberg@haskalah.org>