Some fixes in the IOCP plugin regarding timeouts

- 0 is a valid timeout value. Return INFINITE if there is no pending
  timer

- Set is_active flag on timer to 0 before invoking callback. The
  callback may remove the timer (which is already popped) and re-add it
  again.

Change-Id: I1cd5c41f510347c339439adf0d3d17bf40fcfa7b
Reviewed-on: http://review.couchbase.org/27781
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>