pillowfight.cc: use pthreads

Change-Id: Ie9b4847e255bcb5a29c0566329da598920234a1d
Reviewed-on: http://review.couchbase.org/25111
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>