RF: Make AnnexRepo.set_metadata() work without iteration

[mih]

Analog to function like add/add_ the normal version now just does things, whereas the _ version returns a generator that needs to be iterator over to actually perform anything. The prior behavior was
a constant source of surprise and anger.

Related to #2755

[codecov]

Codecov Report

Merging #3298 into master will increase coverage by 28.52%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master    #3298       +/-   ##
===========================================
+ Coverage   62.51%   91.03%   +28.52%     
===========================================
  Files         100      263      +163     
  Lines       15410    34180    +18770     
===========================================
+ Hits         9633    31117    +21484     
+ Misses       5777     3063     -2714

Impacted Files	Coverage Δ
datalad/support/tests/test_annexrepo.py	96.39% <100%> (ø)
datalad/metadata/tests/test_search.py	93.43% <100%> (ø)
datalad/support/annexrepo.py	87.96% <100%> (+34.59%)	⬆️
datalad/plugin/tests/test_addurls.py	100% <100%> (ø)
datalad/plugin/addurls.py	99.68% <100%> (+81.36%)	⬆️
datalad/metadata/__init__.py	100% <0%> (ø)	⬆️
datalad/downloaders/tests/utils.py	92.3% <0%> (ø)
datalad/customremotes/tests/__init__.py	100% <0%> (ø)
datalad/cmdline/tests/test_helpers.py	100% <0%> (ø)
datalad/support/tests/test_stats.py	100% <0%> (ø)
... and 230 more

---

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[codecov]

Codecov Report

Merging #3298 into master will increase coverage by 25.04%.
The diff coverage is 87.5%.

@@             Coverage Diff             @@
##           master    #3298       +/-   ##
===========================================
+ Coverage   62.51%   87.55%   +25.04%     
===========================================
  Files         100      263      +163     
  Lines       15410    34176    +18766     
===========================================
+ Hits         9633    29922    +20289     
+ Misses       5777     4254     -1523

Impacted Files	Coverage Δ
datalad/plugin/tests/test_addurls.py	59.31% <0%> (ø)
datalad/plugin/addurls.py	72.98% <0%> (+54.65%)	⬆️
datalad/support/tests/test_annexrepo.py	95.36% <100%> (ø)
datalad/metadata/tests/test_search.py	93.43% <100%> (ø)
datalad/support/annexrepo.py	87.01% <100%> (+33.64%)	⬆️
datalad/downloaders/tests/utils.py	92.3% <0%> (ø)
datalad/customremotes/tests/__init__.py	100% <0%> (ø)
datalad/cmdline/tests/test_helpers.py	100% <0%> (ø)
datalad/support/tests/test_stats.py	100% <0%> (ø)
datalad/tests/test_utils.py	96.85% <0%> (ø)
... and 228 more

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[mih]

I suspect this is non-controversial.