Make lock file location configurable

[bpoldrack]

This introduces datalad.locations.locks to specify the location to be
used for (global) lock files. Defaults to the currently used
<user cache dir>/locks.

Closes #5706

[bpoldrack]

Hopefully uncontroversial, so assuming tests get back green - ready to merge from my POV.

[codecov]

Codecov Report

Merging #5740 (b6ac4ca) into master (0b0e7fe) will decrease coverage by 20.67%.
The diff coverage is 100.00%.

❗ Current head b6ac4ca differs from pull request most recent head e9f2fc9. Consider uploading reports for the commit e9f2fc9 to get more accurate results

@@             Coverage Diff             @@
##           master    #5740       +/-   ##
===========================================
- Coverage   90.63%   69.96%   -20.68%     
===========================================
  Files         308      305        -3     
  Lines       42168    42145       -23     
===========================================
- Hits        38220    29485     -8735     
- Misses       3948    12660     +8712     

Impacted Files	Coverage Δ
datalad/downloaders/base.py	50.88% <ø> (-27.72%)	⬇️
datalad/interface/common_cfg.py	100.00% <ø> (ø)
datalad/local/add_readme.py	92.98% <ø> (ø)
datalad/config.py	96.71% <100.00%> (ø)
datalad/core/distributed/clone.py	91.83% <100.00%> (-0.13%)	⬇️
datalad/core/distributed/tests/test_clone.py	97.34% <100.00%> (+0.01%)	⬆️
datalad/local/tests/test_add_readme.py	100.00% <100.00%> (ø)
datalad/plugin/wtf.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/addurls.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/plugin/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
... and 179 more

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[yarikoptic]

Looks good to me. appveyor fails to run on Macs is appveyor issue unrelated to this PR.

FWIW, I would not even mind it in maint which we might kick out some time sooner than master.

[mih]

Thx!