Spotted during a code read. ConnIO unlocks the mutex before attempting to write to the response channel, which could allow another goroutine to call Close() and close the channel, causing a panic.
Fix is to hold the mutex. This won't cause a deadlock because the `select{}` has a `default` case, so we won't block even if the receiver isn't keeping up.
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.
