Use Promise chaining to enforce read/write queue

[jabza]

A problem in node-persist, used by FlatFile, means multiple async read/writes to the same file are allowed (no file-locking) and can and does cause corruption (which may or may not be caught) in higher frequency situations.

This PR uses a Promise chain to ensure only one async read/write is performed at a time. The queue is rather crude, but does guarantee no multiple writes (if only one FlatFile instance is being used on that directory).

In the future it would be nice to have Promise chain per-file, or seek alternatives to node-persist.
However, for now, this solution has been working for my own project and it's probably worth the added protection vs the potential performance loss.

Checklist

• Use a separate branch in your local repo (not master).
• Test coverage is 100% (or you have a story for why it's ok).

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Closes #694

[delucis]

Thanks @jabza! Do you think we could add a layer of abstraction to make this less verbose?

Something like:

this.request = <T extends any = any>(action: () => Promise<T>): Promise<T> => {
  this.requests = this.requests.then(action, action);
  return this.requests
}

So we can use it like this:

result.state = (await this.request(() => this.games.getItem(gameID))) as State;

Or even better in terms of code clarity might be if you created a separate wrapper for get and set, so we could just do:

result.state = (await this.getRequest(gameID)) as State;

What do you think?

[jabza]

Went with your first option. Pushed that change now

[delucis]

Looks good. Thank you!