performance optimizations

[jerch]

#196 revealed several performance smells, that shall be covered here:

• runtime caches
• discourage bulk_update
  • new update impls: merge_update & flat_update
  • integrate new impls with fast_update package
  • BREAKING: revamp settings.py variables as layouted here runtime caches & perf considerations #196 (comment)
  • backend switch for test cases
  • docs about new backends
• finalize single_update
• use using has a lot of using use shall fix apply using allover db interactions #103
• new beta release

Since this changes the way data gets updated by the resolver, we have to release a new beta for it.

Fixes #196.

[jerch]

This PR is stalled until the new update implementations are added to the fast_update package.

[coveralls]

Pull Request Test Coverage Report for Build 16736724808

Details

• 96 of 114 (84.21%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.8%) to 94.927%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
computedfields/resolver.py	74	92	80.43%

Totals
Change from base Build 16602328270:	-0.8%
Covered Lines:	1340
Relevant Lines:	1397

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💛 - Coveralls