Refactor sync/caching

[raucao]

This was one of the main reason for porting the source code to TypeScript. So now it's time to actually refactor and clean up the code that is virtually impossible to quickly debug due to extended Promise chains, some of which span 20+ functions. (Just FYI, for new contributors: when that code was written, Promise wasn't even part of JavaScript itself, but only a half-standardized spec which we used via a utility library).

My proposal would be to use async/await as much as possible instead.

I have created a project, and added all sync-related issues that I could glance in the list. Maybe we can address some or most of those on the way. But it's mostly to get an overview of open issues in that area, and what we can fix or improve either specifically on the way, or by cleaning up code and making it more robust.

https://github.com/remotestorage/remotestorage.js/projects/1