Performance optimizations

[timrae]

This replaces #36 and adds some other fixes!

Execution speed should be much faster now, especially when there are not many changes to synchronize.

• Maintain track cache between different playlists (thanks to @joshrmcdaniel for amazing work on that!)
• Fix incorrect tidal_playlist_is_dirty() implementation
• Remove more redundant API calls
• Avoid unnecessarily spinning up tasks for tracks that were in match failure cache
• Introduce new rate_limit configuration parameter implemented with leaky bucket rate-limiting algorithm
• Where possible, add new tracks to existing playlist instead of erasing the old ones
• Use asyncio multithreading instead of multiprocessing
• When user has large number of spotify playlists, fetch them in parallel instead of one by one

[timrae]

I think I'm pretty much done with this work now, I'll wait for about a week for any review comments before merging.