control/controlknobs,wgengine/magicsock: implement SilentDisco toggle (#10195)

This change exposes SilentDisco as a control knob, and plumbs it down to
magicsock.endpoint. No changes are being made to magicsock.endpoint
disco behavior, yet.

Updates #540

Signed-off-by: Jordan Whited <jordan@tailscale.com>
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -440,6 +440,13 @@ func (de *endpoint) heartbeat() {
 	de.heartBeatTimer = time.AfterFunc(heartbeatInterval, de.heartbeat)
 }
 
+// setHeartbeatDisabled sets heartbeatDisabled to the provided value.
+func (de *endpoint) setHeartbeatDisabled(v bool) {
+	de.mu.Lock()
+	defer de.mu.Unlock()
+	de.heartbeatDisabled = v
+}
+
 // wantFullPingLocked reports whether we should ping to all our peers looking for
 // a better path.
 //