Various small fixes and enhancements (#440)

Various small fixes and enhancements (#440)

* fix edge case where playlist track has invalid albumdetails

* make the audio buffer more dynamic

* fix loop thread safety (fixes issues with cast speakers)

* simplify some code

* simplify player poll task

* extend timeout for spotify token retrieval

* Fix count of PagedItems queries

* Fix relative paths in playlists

* Handle some edge cases in cleanup procedure

* Fix metadata checksum overwrite

* Fix type of mediaitem image

* additional guards for library edit usage

* Fix local artwork retrieval in artist/album folders

* Handle (and log) slow Spotify connection