This commit adds a Timer interface implemented by HashWheelTimer

HashWheelTimer itself is implemented as per the "Hashed and Hierarchical
Timing Wheels" paper: http://www.cs.columbia.edu/~nahum/w6998/papers/sosp87-timing-wheels.pdf

This implementation doesn't involve hierarchical Timing Wheels.

Non-obtrusive commit. Doesn't force anyone to use it :)