Improve support for external request queues

[whalebot-helmsman]

It is continuation of work from https://github.com/scrapy/scrapy/pull/4605

[codecov[bot]]

Codecov Report

Merging #4783 (fa9453d) into master (d7deba7) will decrease coverage by 1.32%. The diff coverage is 94.87%.

:exclamation: Current head fa9453d differs from pull request most recent head b316bb9. Consider uploading reports for the commit b316bb9 to get more accurate results

@@            Coverage Diff             @@
##           master    #4783      +/-   ##
==========================================
- Coverage   84.16%   82.84%   -1.33%     
==========================================
  Files         162      162              
  Lines       10524    10546      +22     
  Branches     1522     1525       +3     
==========================================
- Hits         8858     8737     -121     
- Misses       1406     1549     +143     
  Partials      260      260              

Impacted Files	Coverage Δ
scrapy/pipelines/images.py	27.82% <0.00%> (-0.25%)	:arrow_down:
scrapy/core/downloader/handlers/s3.py	93.47% <50.00%> (+3.89%)	:arrow_up:
scrapy/extensions/feedexport.py	91.08% <81.81%> (-0.58%)	:arrow_down:
scrapy/core/scheduler.py	99.27% <100.00%> (+5.47%)	:arrow_up:
scrapy/exceptions.py	100.00% <100.00%> (ø)
scrapy/http/response/__init__.py	97.46% <100.00%> (+0.06%)	:arrow_up:
scrapy/http/response/text.py	100.00% <100.00%> (ø)
scrapy/pipelines/files.py	71.52% <100.00%> (-0.16%)	:arrow_down:
scrapy/pqueues.py	99.18% <100.00%> (+<0.01%)	:arrow_up:
scrapy/squeues.py	95.50% <100.00%> (+0.15%)	:arrow_up:
... and 15 more

[wRAR]

Is this ready for reviewing or WIP?

[whalebot-helmsman]

It is ready for review

[whalebot-helmsman]

Hey @Gallaecio I addressed both of your comments

[kmike]

I left a couple of miinor comments, but the PR looks good, thanks @whalebot-helmsman !