wgengine/magicsock: implement relayManager handshaking (#15977)

CallMeMaybeVia reception and endpoint allocation have been collapsed to
a single event channel. discoInfo caching for active relay handshakes
is now implemented.

Updates tailscale/corp#27502

Signed-off-by: Jordan Whited <jordan@tailscale.com>

wgengine/magicsock/relaymanager_test.go

@@ -8,6 +8,7 @@ import (
 	"testing"
 
 	"tailscale.com/disco"
+	"tailscale.com/types/key"
 )
 
 func TestRelayManagerInitAndIdle(t *testing.T) {
@@ -16,11 +17,11 @@ func TestRelayManagerInitAndIdle(t *testing.T) {
 	<-rm.runLoopStoppedCh
 
 	rm = relayManager{}
-	rm.cancelOutstandingWork(&endpoint{})
+	rm.stopWork(&endpoint{})
 	<-rm.runLoopStoppedCh
 
 	rm = relayManager{}
-	rm.handleCallMeMaybeVia(&disco.CallMeMaybeVia{})
+	rm.handleCallMeMaybeVia(&endpoint{c: &Conn{discoPrivate: key.NewDisco()}}, &disco.CallMeMaybeVia{ServerDisco: key.NewDisco().Public()})
 	<-rm.runLoopStoppedCh
 
 	rm = relayManager{}