Change track.last_modified to be milliseconds since Unix epoch

[jodal]

To be consistent with playlist.last_modified.

[adamcik]

We could probably just go ahead and do this, only think I suspect actually cares about this value is the local scanner.

[tkem]

@adamcik: Mopidy-Local-SQLite uses track.last_modified for its "Last Week's Updates" virtual directory, so changing the resolution to milliseconds would break this.
That being said, sure that's easy to fix in Mopidy-Local-SQLite, and consistency might be more important.