Wait with this until it becomes necessary or we can achieve same performance as libspotify
This reverts commit 997b4da4ad8edf299f2437f49f0a8e7337cfa658 and 2da993cc7b49dba28ae7e5db461aee973919e6c9
The web api might be accessed from different threads (library, worker,
dacp), therefor protect from concurrently running refresh-token requests
(accessing the globals in spotify_webapi.c)
This avoids adding unplayable items to the library and due to the active
track relinking should make additional tracks playable by linking to a
playable version of the track.
forked-daapd does not support a 1:n relationship between tracks and
artists. Just take the first artist the spotify web api returns
(libspotify does also return only one artist, so this should not
introduce a regression).
