Async refresh

Move refresh of google music content to an extra thread.
This fixes two problems:

1. Very long startup times on small systems, like the raspberry pi
2. Remote changes to the content, like new/changed playlists
Well, it's just that, on my raspberry it takes ages to sync playlists. I wanted to make sure, that everything else is done by the time we start syncing.
But it's kind of a magic number.