IOCP: Handle async 0-interval timers better

Change-Id: I82d49a06afd19dece29f78129805f07f2c6e8796
Reviewed-on: http://review.couchbase.org/31797
Reviewed-by: Brett Lawson <brett19@gmail.com>
Tested-by: Mark Nunberg <mnunberg@haskalah.org>