Optimisation: do not re-access mapper properties inside the request loop

[ml-evs]

Closes #1219 by accessing the config variables outside of the response field loop.

[codecov]

Codecov Report

Merging #1223 (145a0aa) into master (991591b) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 145a0aa differs from pull request most recent head e591e18. Consider uploading reports for the commit e591e18 to get more accurate results

@@            Coverage Diff             @@
##           master    #1223      +/-   ##
==========================================
- Coverage   91.28%   91.27%   -0.02%     
==========================================
  Files          72       72              
  Lines        4292     4287       -5     
==========================================
- Hits         3918     3913       -5     
  Misses        374      374              

Flag	Coverage Δ
project	91.27% <100.00%> (-0.02%)	⬇️
validator	90.66% <100.00%> (-0.02%)	⬇️

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Impacted Files	Coverage Δ
...made/server/entry_collections/entry_collections.py	97.72% <100.00%> (+0.03%)	⬆️
optimade/server/routers/info.py	95.45% <0.00%> (-0.20%)	⬇️
optimade/server/config.py	91.39% <0.00%> (-0.19%)	⬇️
optimade/server/entry_collections/elasticsearch.py	97.40% <0.00%> (-0.13%)	⬇️
optimade/server/routers/utils.py	98.13% <0.00%> (ø)
optimade/server/exception_handlers.py	84.28% <0.00%> (ø)

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[JPBergsma]

LGTM, I wonder how many other cases there are in our code where we call a function within a loop where calling it once outside the loop would suffice.
On second thought, should we not change the case of the ALL_ATTRIBUTES, SUPPORTED_PREFIXES and perhaps some of the other methods of the BaseResourceMapper to small case ? They are not really constants, so I do not think they should be in capitals.

[ml-evs]

LGTM, I wonder how many other cases there are in our code where we call a function within a loop where calling it once outside the loop would suffice.

This is the only place I could spot these being used, we could do some better caching but it would probably require breaking changes.

On second thought, should we not change the case of the ALL_ATTRIBUTES, SUPPORTED_PREFIXES and perhaps some of the other methods of the BaseResourceMapper to small case ? They are not really constants, so I do not think they should be in capitals.

I'm not sure its worth breaking backwards-compatibility for, but I agree that there is not really a good reason why they are capitalised... I guess they should be constant for a particular running server, at least (which was probably our rationale).