RJS-2784: Clear internal state to avoid crashes when React Native app is reloaded

[kneth]

What, How & Why?

This closes #6579

During development it is common to reload a React Native app. As it is a rough process, it is important to clear the previous state - including listeners. Realm Core provides some low-level method to clear many internal data structures, and by invoking these when the SDK is initialized (loaded in the app), any state from the previous app instance is removed.

Testing is difficult, and I have only tried it manually in a app running locally on my computer.

☑️ ToDos

• [x] 📝 Changelog entry
• [ ] 📝 Compatibility label is updated or copied from previous entry
• [ ] 📝 Update COMPATIBILITY.md
• [ ] 🚦 Tests
• [ ] 📦 Updated internal package version in consuming package.jsons (if updating internal packages)
• [ ] 📱 Check the React Native/other sample apps work if necessary
• [ ] 💥 Breaking label has been applied or is not necessary

[kneth]

Coverage tests crashed with "free(): invalid pointer", and we should investigate it