performance improvement: create only one instance of _Filterer

[tyarimi]

_Filterer is stateless, so no need to initialize it on every call to filter_applies. It cuts the filtering time by ±30%.

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This change is 

[codecov]

Codecov Report

Merging #674 into develop will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #674   +/-   ##
========================================
  Coverage    95.44%   95.44%           
========================================
  Files           19       19           
  Lines         3558     3559    +1     
========================================
+ Hits          3396     3397    +1     
  Misses         162      162           

Impacted Files	Coverage Δ
mongomock/filtering.py	98.96% <100.00%> (+<0.01%)	⬆️

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[tyarimi]

@pcorpet can you please review and merge this PR? it improves performance dramatically.

[pcorpet]

Thanks for the proposal. Note that the Filterer object was introduced for adding new features (that haven't made it yet to the main branch) and that I'll probably have to roll it back later on. As for now, if you're indeed seeing a +30% for performances, it's certainly worth it.

[tyarimi]

@pcorpet thanks for merging the PR! I will try to find some time to introduce more performance improvements to the filtering logic, as we have thousands of tests depending on it.