Cache scrobbels and submit when online again

[0nse]

When listening to music offline, the scrobbler naturally can't submit tracks. Likewise, when my Internet connection drops, the client stops submitting scrobbles until I restart mopidy.

Scrobbled tracks should be cached and submitted the next time the scrobbler connects to Last.FM.

[ArthaTi]

Issue from 2016, but heavily affects me as I, in my current setup, I have to rely on tethering network from my phone. I often disconnect and having to restart Mopidy every time is annoying, especially since it takes a while.

[djmattyg007]

@Soaku @0nse Please take a look at https://github.com/djmattyg007/mopidy-advanced-scrobbler

[djmattyg007]

Also there's a PR awaiting review for the Mopidy-Spotify extension that improves the startup time:

mopidy/mopidy-spotify#312