Improved logic for checking overflow workers

[comtihon]

As discussed in #83 (comment) I made improvements in zugolosian`s changes.
In zugolosian#1 all commits are discussed.
Summing up:

• overflow rip check is now hold periodically without mass timer creation
• use monotonic time instead of os:timestamp
• add improvement in case of lifo when checking old overflow workers

[devinus]

@fishcakez I could really use another set of eyes on this here.

[fishcakez]

When reaping in batches there is no need to read/store the monotonic time in the workers queue, only the time of the poll when it would be reaped or the time of the last reap. This need not be a clock time but could be a counter that increments once per reap, in a similar way to a timer wheel works. Using a counter (instead of monotonic time) may introduce some lag in the TTL but I don't think that will be an issue here because strict timing is not required. This would remove the OTP-18 requirement and should be faster. With OTP-18 strict timing would be possible using absolute timers.

This branch needs to access both ends of the worker queue so you may want to consider a different data structure. The fifo strategy already does a copy of the queue when inserting a worker.

[fishcakez]

I am not sure how I feel about adding features to poolboy because I think its main feature is that it is simple. Part of me feels that poolboy would benefit from removing the overflow feature because churn occurs surprisingly often and I advise all Ecto/Postgrex users not to set max_overflow. The churn means that workers need to created/destroy resources frequently which is more costly than the gains from dynamic resizing. If there isn't a resource and poolboy is just being used to limit concurrency then satefyvalve, jobs or sregulator would be more appropriate tools.