The TelemetryClient only flushed at 50 events, so the server silently
lost all queued telemetry on restart. Add startPeriodicFlush/stop methods
to TelemetryClient, wire up 60s periodic flush in server initTelemetry,
and flush on SIGTERM/SIGINT before exit.
Co-Authored-By: Paperclip <noreply@paperclip.ing>