Performance optimization / refactoring (#43)

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
* More typing hints / typing fixes

example_config.yml

@@ -14,6 +14,6 @@ spotify:
 #excluded_playlists:
 #  - spotify:playlist:1ABCDEqsABCD6EaABCDa0a
 
-# number of concurrent subprocesses when searching tracks in a playlist.
-# increasing this value can improve sync speed, but may increase 429 errors
-subprocesses: 25
+# increasing these parameters should increase the search speed, while decreasing reduces likelihood of 429 errors
+max_concurrency: 10 # max concurrent connections at any given time
+rate_limit:      12 # max sustained connections per second