Hashed Wheel Timer

Simple hashed wheel timer. See http://www.cs.columbia.edu/~nahum/w6998/papers/sosp87-timing-wheels.pdf

TODO:

1. Add Size to inspect the size the WheelTimer.
2. Add Length to Node inspect the length of the linked list.
3. Allow scheduling ticks outside of the immediate interval.
4. Support at-least concurrent writes (multi-producer, single consumer).