pillowfight.cc: extract pool object out of thread context

It allows to share pool among multiple threads

Change-Id: Ib1f8bf6960f554afa3bd0678b19828df3a64b6fe
Reviewed-on: http://review.couchbase.org/25104
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>