Implement cb_thread_equal for win32

Change-Id: Idafb2cf17d276e446277c76725cf8dd84267b366
Reviewed-on: http://review.couchbase.org/30130
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>