TUN-3869: Improve reliability of graceful shutdown.

- Don't rely on edge to close connection on graceful shutdown in h2mux, start muxer shutdown from cloudflared.
- Don't retry failed connections after graceful shutdown has started.
- After graceful shutdown channel is closed we stop waiting for retry timer and don't try to restart tunnel loop.
- Use readonly channel for graceful shutdown in functions that only consume the signal

connection/http2.go

@@ -39,7 +39,7 @@ type http2Connection struct {
 
 	activeRequestsWG  sync.WaitGroup
 	connectedFuse     ConnectedFuse
-	gracefulShutdownC chan struct{}
+	gracefulShutdownC <-chan struct{}
 	stoppedGracefully bool
 	controlStreamErr  error // result of running control stream handler
 }
@@ -52,7 +52,7 @@ func NewHTTP2Connection(
 	observer *Observer,
 	connIndex uint8,
 	connectedFuse ConnectedFuse,
-	gracefulShutdownC chan struct{},
+	gracefulShutdownC <-chan struct{},
 ) *http2Connection {
 	return &http2Connection{
 		conn: conn,