pillowfight.cc: allow to share IO object in single-threaded mode only

Change-Id: Ib4395624cdb4a710b391d89a50cf3a2b2bb6ad35
Reviewed-on: http://review.couchbase.org/25105
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>