Adds support to Coordination to call SetAllPeersLost() when it is closed. This ensure that when we disconnect from a Coordinator, we set all peers lost.
This covers CoderSDK (CLI client) and Agent. Next PR will cover MultiAgent (notably, `wsproxy`).
Fixes an issue where remote forwards are not correctly torn down when using OpenSSH with `coder ssh --stdio`. OpenSSH sends a disconnect signal, but then also sends SIGHUP to `coder`. Previously, we just exited when we got SIGHUP, and this raced against properly disconnecting.
Fixes https://github.com/coder/customers/issues/327
