Fix HashedWheelTimer to properly handle timeouts that are multiple of wheel period

see ticket: https://www.assembla.com/spaces/akka/tickets/2898-hashedwheeltimer-incorrectly-schedules-events-when-delay-is-a-multiple-of-the-wheel-period#/activity/ticket:

applies to both 2.1.0 and 2.0.5 (although HWT source is in a different location in 2.0.5)