Implement a HashWheel timer

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.
 Add tests for pause and resume
Remove `timer` from some docs on registration

IntelliJ accidentally renamed some occurrences of Registration
to TimerRegistration in @literal docstrings and comments.
This commit revers these changes.
Merge branch 'master' into enchancement-hash-wheel-timer

Conflicts:
	reactor-core/src/test/groovy/reactor/core/HashWheelTimerSpec.groovy
Merge branch 'master' into enchancement-hash-wheel-timer
Groovy drifts even more than 50ms sometimes, increasing default to 100...
Add missing timer cancellation
Merge pull request #272 from ifesdjeen/enchancement-hash-wheel-timer

Implement a HashWheel timer