Use a "global" playlist mapping in MPD dispatch code

[adamcik]

https://discuss.mopidy.com/t/mpc-clients-are-slow-to-connect-to-git-version-of-mopidy/518 has some of the backstory. But the short version is that redoing this mapping for users with many spotify playlists is costly (and can lead to inconsistencies).

So we should be caching this or just moving it up to the server level instead of doing this once per session / connection.

[jodal]

Fixed by PR #968