Fix polling order in contrib modules

[my8100]

This PR use project_priority_map to store (priority, -timestamp) as value,
so as to respect both job priorities across projects and the FIFO principle.

See also issue #187 and PR #344.

[codecov]

Codecov Report

Merging #349 into master will increase coverage by 1.08%.
The diff coverage is 85.48%.

@@            Coverage Diff             @@
##           master     #349      +/-   ##
==========================================
+ Coverage   68.37%   69.45%   +1.08%     
==========================================
  Files          17       20       +3     
  Lines         860      920      +60     
  Branches      104      115      +11     
==========================================
+ Hits          588      639      +51     
- Misses        242      246       +4     
- Partials       30       35       +5

Impacted Files	Coverage Δ
scrapyd/utils.py	77.5% <100%> (+0.57%)	⬆️
scrapyd/contrib/fix_poll_order/spiderqueue.py	100% <100%> (ø)
scrapyd/app.py	81.81% <40%> (-4.46%)	⬇️
scrapyd/contrib/fix_poll_order/poller.py	75% <75%> (ø)
scrapyd/contrib/fix_poll_order/sqlite.py	92.85% <92.85%> (ø)

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[my8100]

@Digenis
How about this contrib implementation?

[Digenis]

I was busy, I'll review it in a day or two

[my8100]

Any progress?

[jpmckinney]

Should do something similar to Launcher, etc. so that users can select any module (including their own), not just between these two modules.

[jpmckinney]

This looks similar to #344 except is puts the new logic in a contrib module, and users opt-in to using that module.

As mentioned there, this multi-queue prioritization approach seems more complicated than a single queue, discussed in #187. I've now made the poller configurable, so a user could copy this code into their own project, and load it, if they prefer this approach.