pillowfight: Unify thread contexts and instances

This makes the specific instance bound to its ThreadContext object,
allowing less locking contention and a more direct association between
the instance and the thread.

Change-Id: I9c0d3009eb0302e5351dea1a66be1f6910a5234e
Reviewed-on: http://review.couchbase.org/41969
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>