Fix: whereField creates a memory leak

[milaGGL]

fix: #12613
revert the thread safe memoizer introduced in this pr: #11781

@cherylEnkidu :
This PR intends to fix the data race and memory leak issue.

When using memorized field, it leads to data race. However, the solution for fixing data race issue introduces memory leak.

So this PR construct a flatten filters for composite filter during initialization.

The reason for not providing GetFlattenFilter() in base filter class is creating flatten filter in field filter constructor will cause infinity loop.

Since GetFlattenFilter() is widely used in query calls, init flatten filters vector in composite filter during construction won't hurt efficiency a lot comparing to construct flatten filters vector upon request.

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestoreInternal.framework

  Overall coverage changed from 88.14% (9be3928) to 88.20% (77ff64d) by +0.06%.

  Filename	Base (9be3928)	Merge (77ff64d)	Diff
  composite_filter.cc	90.10%	87.76%	-2.34%
  exception.cc	23.68%	84.21%	+60.53%
  field_filter.cc	95.00%	94.55%	-0.45%
  filter.cc	70.00%	62.50%	-7.50%
  leveldb_key.cc	99.02%	98.63%	-0.39%
  leveldb_mutation_queue.cc	93.56%	92.42%	-1.14%
  local_serializer.cc	87.78%	88.33%	+0.56%
  query_core.cc	95.80%	95.91%	+0.11%
  target.cc	96.23%	96.35%	+0.12%

Test Logs

• Base (9be3928): https://github.com/firebase/firebase-ios-sdk/actions/runs/9193063532
• Merge (77ff64d): https://github.com/firebase/firebase-ios-sdk/actions/runs/9195730048

1. https://storage.googleapis.com/firebase-sdk-metric-reports/V7ZyS0seKZ.html

[cherylEnkidu]

Hi @ehsannas , could you please review this fix?

[ehsannas]

The reason for not providing GetFlattenFilter() in base filter class is creating flatten filter in field filter constructor will cause infinity loop.

why? maybe you shouldn't call GetFlattenedFilters() in the constructor, but should still have GetFlattenedFilters() defined for field filters [which just returns *this].