No longer cache tracks in SPSession

Change-Id: I9c8f99ff649aa70fbd2488b2a9d9feb5e7d83bdd
No longer completely rebuild -loadedIndexes in SPSpareList

Rebuilding the whole index set every time something changed caused huge memory spikes under heavy load.

Change-Id: I1639b6cc1f94019531908ff3dc7862971e5cde45
Offline Playlists: Keep track of which playlist items are loading

Change-Id: Ibac4e39e74c623ffc10988fd9cddd66661a02dcc