feat(2.0, nodejs): Silently request to stop background syncing when WalletMethodHandler is dropped

[marc2332]

Description of change

This will make sure that the background syncing is stopped when the WalletMethodHandler is dropped. You might ask why this is useful as there is already destroyWallet method. Well, this is more like a "fallback" plan for when that method is not called yet the wallet is dropped. For example, I encountered that everytime I edited some frontend code in Firefly, that triggered webpack to reload the frontend, and thus removing from memory all the Wallets instances, without their stopBackgroundSync method called. This caused that if I wanted to log into a profile whose stopBackgroundSync method was not called before, now gave me a rocksdb error, because, the old wallet instance was still living in the background syncing thread, and thus the DB still locked.

Type of change

• Enhancement

How the change has been tested

In firefly, but you can test it by starting the background syncing and never stopping, but destroying the wallet.

Change checklist

Tick the boxes that are relevant to your changes, and delete any items that are not.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I have checked that new and existing unit tests pass locally with my changes

[marc2332]

A potential case where a new Wallet instance is created quickly enough to throw a no available locks error, just after a previous wallet instance has been dropped without manually calling stopBackgroundSync and thus calling it asynchronously when it drops will be improved by #1797

[thibault-martinez]

Please rebase to remove the python errors and apply format 🙏🏻

[thibault-martinez]

Thank you 🙏🏻