wgengine/magicsock: re-shape relayManager to use an event loop (#15935)

The event loop removes the need for growing locking complexities and
synchronization. Now we simply use channels. The event loop only runs
while there is active work to do.

relayManager remains no-op inside magicsock for the time being.
endpoints are never 'relayCapable' and therefore endpoint & Conn will
not feed CallMeMaybeVia or allocation events into it.

A number of relayManager events remain unimplemented, e.g.
CallMeMaybeVia reception and relay handshaking.

Updates tailscale/corp#27502

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

wgengine/magicsock/relaymanager_test.go

@@ -0,0 +1,29 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+package magicsock
+
+import (
+	"net/netip"
+	"testing"
+
+	"tailscale.com/disco"
+)
+
+func TestRelayManagerInitAndIdle(t *testing.T) {
+	rm := relayManager{}
+	rm.allocateAndHandshakeAllServers(&endpoint{})
+	<-rm.runLoopStoppedCh
+
+	rm = relayManager{}
+	rm.cancelOutstandingWork(&endpoint{})
+	<-rm.runLoopStoppedCh
+
+	rm = relayManager{}
+	rm.handleCallMeMaybeVia(&disco.CallMeMaybeVia{})
+	<-rm.runLoopStoppedCh
+
+	rm = relayManager{}
+	rm.handleBindUDPRelayEndpointChallenge(&disco.BindUDPRelayEndpointChallenge{}, &discoInfo{}, netip.AddrPort{}, 0)
+	<-rm.runLoopStoppedCh
+}