last.fm: update to protocol version 1.2

v1.2 of the submission protocol uses a different authentication
scheme, has a different set of response messages, and doesn't
give any direction about delays between track submissions.

It also adds a 'now playing' submission, which we send from the
regular timeout (limiting it to once per 15 seconds).

Track submissions now include a 'source' parameter, which we don't
properly support yet.

We also implement the current recommendations for delays between
handshake requests when the handshake fails.