Defer creation of current-time-in-seconds thread until runtime

[tjquinno]

Replaces #3374
Resolves #3389

This PR picks up Tomas's earlier one (#3374) and incorporates Daniel's comments.

The separate commits divide up the changes clearly, I hope.

Aside from adopting Daniel's suggestion for a three-state model for the executor, the other main change is to maintain a list of ExponentiallyDecayingReservoirs for which their current-time-in-seconds should be updated periodically, rather than keep a list of Runnable; we don't need to be that general and it's clearer and probably faster this way.

Also, the earlier shutdown processing (awaitTermination to wait for the scheduled tasks to finish) seemed to time out rather than complete, so I've changed it to use shutdownNow instead.

[tjquinno]

Comment copied from the other PR:

danielkec yesterday Member
Seems to me we need one more atomic state here to safeguard against init() being invoked just after onServerShutdown() gate eg. 'INIT, STARTED, STOPPED'.

[tjquinno]

The refactoring in the previous push takes the single-thread executor handling out of ExponentiallyDecayingReservoir because in a future PR the ConcurrentGauge will also use this mechanism. State checking and management occurs entirely within synchronized methods; none of the critical regions should execute very often so performance should not really be an issue.

[tomas-langer]

LGTM