Blocked callers waiting on a pingTracker response channel would
hang forever if the server Close()d mid-probe. Drain the pending map on
Close so those goroutines unblock and exit cleanly.
Updates #3157
Implement tailcfg.PingRequest support so the control server can verify
whether a connected node is still reachable. This is the foundation for
faster offline detection (currently ~16min due to Go HTTP/2 TCP retransmit
behavior) and future C2N communication.
The server sends a PingRequest via MapResponse with a unique callback
URL. The Tailscale client responds with a HEAD request to that URL,
proving connectivity. Round-trip latency is measured.
Wire PingRequest through the Change → Batcher → MapResponse pipeline,
add a ping tracker on State for correlating requests with responses,
add ResolveNode for looking up nodes by ID/IP/hostname, and expose a
/debug/ping page (elem-go form UI) and /machine/ping-response endpoint.
Updates #2902
Updates #2129
