Make backups and population asynchronously parallel

[jwodder]

Closes #173.

This adds a -w/--workers option to the update-from-backup, populate, and populate-zarrs commands; the option can also be set via the config file.

[codecov[bot]]

Codecov Report

Merging #205 (eafbec4) into draft (b1b54bd) will increase coverage by 2.57%. The diff coverage is 79.19%.

@@            Coverage Diff             @@
##            draft     #205      +/-   ##
==========================================
+ Coverage   76.82%   79.40%   +2.57%     
==========================================
  Files           8       13       +5     
  Lines        1450     1777     +327     
  Branches      233      299      +66     
==========================================
+ Hits         1114     1411     +297     
+ Misses        260      257       -3     
- Partials       76      109      +33     

Impacted Files	Coverage Δ
tools/backups2datalad/__main__.py	62.28% <52.38%> (+8.11%)	:arrow_up:
tools/backups2datalad/datasetter.py	71.16% <69.81%> (-1.28%)	:arrow_down:
tools/backups2datalad/adandi.py	76.31% <76.31%> (ø)
tools/backups2datalad/zarr.py	81.27% <80.00%> (+1.34%)	:arrow_up:
tools/backups2datalad/asyncer.py	81.22% <84.52%> (-0.50%)	:arrow_down:
tools/backups2datalad/logging.py	85.29% <85.29%> (ø)
tools/backups2datalad/adataset.py	85.47% <85.47%> (ø)
tools/backups2datalad/aioutil.py	86.50% <86.50%> (ø)
tools/backups2datalad/annex.py	90.32% <89.47%> (-0.86%)	:arrow_down:
tools/backups2datalad/config.py	91.30% <91.30%> (ø)
... and 6 more

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[yarikoptic]

thanks. current one crashed due to #207 , so let's just merge and try this one!