pillowfight: threads should always access their own items

This fixes an issue where not-found errors were received when one thread
tried to access an item not yet set by another thread.

Change-Id: I9fd10b4d5aba9d62c1299813131873789d6ee32e
Reviewed-on: http://review.couchbase.org/42127
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>