Make SchemaController and SchemaCache stateless

[PiotrZawadzki-TomTom]

New Feature / Enhancement Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.

Current Limitation

Currently Schema-controller, database-controller and schema cache is a singleton and it makes whole parse-server statefull

Feature / Enhancement Description

The concept is to make schema logic stateless and make possible to override in parse-configuration schema cache, so that it is possible to e.g cache it in Redis or another custom solution

Example Use Case

We are going to run multitenant parse server with cloud code. Due to limitations of Cloud code (which we have a lot in our application) we cannot just run multiple instances of parse-server in single node process. We are running single parse instance but we have overridden database adapter to choose appropriate mongo-url every call (we are using CLS to have consistent request objects). The same we have done with caches and thanks to this PR it is possible to apply this strategy to parse schema cache.

Alternatives / Workarounds

NA

3rd Party References

NONE

[parse-github-assistant]

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