Fixes an issue where a MultiAgentConn isn't closed properly when the coordinator it is connected to is closed.
Since servertailnet checks whether the conn is closed before reinitializing, it is important that we check this, otherwise servertailnet can get stuck if the coordinator closes (e.g. when we switch from AGPL to PGCoordinator after decoding a license).
This one is huge, and I'm sorry.
The problem is that once I change `tailnet.Conn` to start doing v2 behavior, I kind of have to change it everywhere, including in CoderSDK (CLI), the agent, wsproxy, and ServerTailnet.
There is still a bit more cleanup to do, and I need to add code so that when we lose connection to the Coordinator, we mark all peers as LOST, but that will be in a separate PR since this is big enough!
Adds support for graceful disconnect to PGCoordinator. When peers gracefully disconnect, they send a disconnect message. This triggers the peer to be disconnected from all tunneled peers.
The Multi-Agent Client supports graceful disconnect, since it is in memory and we know that when it is closed, we really mean to disconnect.
The v1 agent and client Websocket connections do not support graceful disconnect, since the v1 protocol doesn't have this feature. That means that if a v1 peer connects to a v2 peer, when the v1 peer's coordinator connection is closed, the v2 peer will
see it as "lost" since we don't know whether the v1 peer meant to disconnect, or it just lost connectivity to the coordinator.
re: #10528
Refactors PG Coordinator to work with the Tailnet v2 API, including wrappers for the existing v1 API.
The debug endpoint functions, but doesn't return sensible data, that will be in another stacked PR.
Spike Curtis