When an agent receives a node, it responds with an ACK which is relayed
to the client. After the client receives the ACK, it's allowed to begin
pinging.
Adds support to `ServerTailnet` to set all peers lost before attempting to reconnect to the coordinator. In practice, this only really affects `wsproxy` since coderd has a local connection to the coordinator that only goes down if we're shutting down or change licenses.
