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
Split library and playlist refresh

* split library refresh from playlist refresh
* use some more sane method to schedule refresh
Increment play counts

Increment play counts for played tracks if 2/3 of track where played
Move checkiing to one place
Merge branch 'play-count' into all-features

Conflicts:
	mopidy_gmusic/session.py
Merge branch 'async-refresh' into all-features

Conflicts:
	mopidy_gmusic/__init__.py
	mopidy_gmusic/ext.conf
	tests/test_extension.py