Reset maximum skip limit

[jcass77]

When more than five tracks in a row are unplayable, the 'maximum skip limit' is enforced to avoid the client from exceeding Pandora's internal request thresholds.

This works as expected, but the limit is never reset again once it has been reached, and the only way to resume playback is to restart Mopidy. Log contains:

2016-01-26 06:07:28,518 - WARNING  Maximum track skip limit (5) exceeded.
2016-01-26 06:07:28,521 - WARNING  Track is not playable: pandora:track:788279271243133277:4b93698ca1d1b60a1e496911cad2e191a0645cda5c3ab18ebcc5328266209b4b2fe04a60810b426142cc58b1431e06c45e5d5323ac6195f3

A sensible point at which to reset the counter might be when the user does a manual refresh in the web client, or switches to a different station.

[jcass77]

The rest should now occur whenever the user browses to another folder.