feat: unregister peer asap on shutdown

[VinozzZ]

Which problem is this PR solving?

In order to recalculate new destination for traces during shutdown as soon as possible, we need to announce peer unregistration first thing.

Short description of the changes

• use the done channel from catching the termination signal in main.go in peer management

[kentquirk]

Code-wise, this is fine -- if it's actually what we want.

I do think we should tell our peers to stop sending to us immediately, but we might need to do it with a "pause" message or something like that -- because we ALSO need to be draining spans by sending them to the other peers. If we drop out of the peer list entirely, then we have no way to forward spans anymore.

So I think we need to add a way to indicate that a peer is still alive but should not be a target of new traffic.

[kentquirk]

👍