Akka-style message Scheduler

[akatashev]

First of all, thanks for Pykka, it's really nice!
There is this issue about auto-scheduling: #53

I needed this functionality for my project as well, because I believe, that an ability to use schedulers to retransmit messages, schedule job executions and so on, is quite a bit part of Akka, so I created it for myself and I hope, it could be useful for Pykka community as well.

Basically, I implemented the simplest versions of Cancellable, Scheduler.scheduleOnce, Scheduler.scheduleWithFixedDelay and Scheduler.scheduleAtFixedRate from Akka.

There are 3 schedulers for 3 different kinds of Actors:
ThreadingScheduler - based on threading.Timer.
EventletScheduler - based on eventlet.spawn_after.
GeventScheduler - based on gevent.spawn_later.

[akatashev]

I believe, that's possibly a typo:

pykka/pykka/eventlet.py

Line 43 in 4c4c9a7

self.event.wait()

Neither of EventletEvent and eventlet.event.Eventlet has event attribute. So, probably, it should be self.wait() or super().wait().
Didn't try to "fix" it, because I'm not sure what exactly it should be.

Meanwhile flake8 is angry about _future.pyi that I didn't touch. Other checks pass.

[codecov]

Codecov Report

Merging #95 into develop will increase coverage by 0.51%.
The diff coverage is 99.00%.

@@             Coverage Diff             @@
##           develop      #95      +/-   ##
===========================================
+ Coverage    95.37%   95.88%   +0.51%     
===========================================
  Files           13       14       +1     
  Lines          562      656      +94     
===========================================
+ Hits           536      629      +93     
- Misses          26       27       +1     

Impacted Files	Coverage Δ
pykka/_scheduler.py	98.68% <98.68%> (ø)
pykka/_threading.py	96.61% <100.00%> (+0.38%)	⬆️
pykka/eventlet.py	91.17% <100.00%> (+0.85%)	⬆️
pykka/gevent.py	100.00% <100.00%> (ø)

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[djmattyg007]

The code in this PR that calls time.time() really should be updated to use a monotonic timer.

[lostcontrol]

@jodal any plan on having this merged at some point? This is also a much needed feature. There are workarounds but would be nice to have something in Pykka directly.

Still, it would be nice to have it playing nicely with proxies also.

[akatashev]

@lostcontrol I believe, this PR is extremely outdated, Pykka 3 is pretty different to the version that I had, when I was working this PR. It would need heavy refactoring. And there is a number of possible improvements, like the one, that @djmattyg007 mentioned. I'd be happy to refactor it, improve it and resolve all the conflicts, but I am not sure whether it makes any sense since there is no reaction from @jodal on any of opened PRs.