fix(cloud_firestore): Fix crashes on iOS/macOS

[paulw11]

Description

This PR changes the FLTFirebaseFirestorePlugin.serverTimestampMap property from an instance of NSMutableDictionary to NSCache. NSCache is inherently thread-safe while NSMutableDictionary is not.

Additionally, this property was not initialised in a consistent manner. It is a static property, but was being initialised in an instance init function as well as opportunistically in the property getter.

The property is now instantiated via a dispatch_once block in the getter.

All access to the property is now via the getter.

Related Issues

Fix flutter/#10715

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• My PR includes unit or integration tests for all changed/updated/fixed behaviors (See Contributor Guide).
• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

[russellwheatley]

Hey @paulw11, thanks for the fix! Appreciated. To get this over the line, could you merge with master branch? You might have to run melos run format for the project. This would require installing melos which you can do with this command:

dart pub global activate melos

I think this ought to fix CI issues but will see what the result is 😄