Standarize `vitals` signature; Add ts info to sources

* Standardize on `(err,vitals)` as a callback signature from `vitals`
  functions in master stream, source and SocketSource.
* Master: Add our own rewind status into the slave status response
* Source: Add `last_ts` to status return for Icecast and Transcoding, to
  help track down where we're losing time