Properly handle completion to the tracker.

[jlouis]

We are currently not sending completed messages to the tracker, which is utterly wrong. We should fix this ASAP.

[jlouis]

Things to figure out:

• Where do we recognize that we go from being a seeder to begin a leecher?
• Who should message who to get this done?
• Is there any sanity checking to execute?

[jlouis]

This is cooking now. Also, when it goes in, you should bump the protocol version.