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
Unit tests for Change (IsEmpty, Merge, Type, PingNode constructor),
ping tracker (register/complete/cancel lifecycle, concurrency, latency),
and end-to-end servertests exercising the full round-trip with real
controlclient.Direct instances.
Updates #2902
Updates #2129