Timer with duration longer than num-slots * tick-duration fails silently

[DarrenTsung]

Recently tried to use this timer for expiring a connection that was alive for over an 1hr 5mins (3,900,000ms), which is larger than the default values for num-slots (4,096 by default) and tick-duration (100ms by default).

Tested behavior was that the timer did not go off and notify the Future properly.

Possible solutions:

1. Return Result<()> for sleep(), timeout(), etc. where logic is checked such that duration <= num-slots * tick-duration.
2. Dynamically change num-slots or tick-duration (whichever makes sense) such that duration is scheduled correctly.

Any thoughts on the right way to proceed? Right now I can just adjust the tick-duration to 1000ms as I don't need precision, and I think I can work on a PR next week.