async StoredUserList saving

[wuangg]

Offload JSON serializing and file writing to another thread so as not to put a strain on the main thread performance when interacting with user lists that contain a lot of list entries.

[ghost]

based

[wuangg]

I have acknowledged about using a single thread executor for this right after I created this PR. I will do the required changes once I have free time.

[wuangg]

Each StoredUserList now has a single thread executor on their own, these executors will be shut down once the server is stopped.

[electronicboy]

I would just use a shared executor between all of them, no need for them to each have their own

[wuangg]

Yes, I should have gone with that solution as I wasn't thoughtful enough earlier. Thanks for the heads up.