TUN-5204: Unregister QUIC transports on disconnect

This adds various bug fixes when investigating why QUIC transports were
not being unregistered when they should (and only when the graceful shutdown
started).

Most of these bug fixes are making the QUIC transport implementation closer
to its HTTP2 counterpart:
 - ServeControlStream is now a blocking function (it's up to the transport to handle that)
 - QUIC transport then handles the control plane as part of its Serve, thus waiting for it on shutdown
 - QUIC transport now returns "non recoverable" for connections with similar semantics to HTTP2 and H2mux
 - QUIC transport no longer has a loop around its Serve logic that retries connections on its own (that logic is upstream)

connection/control.go

@@ -29,7 +29,9 @@ type controlStream struct {
 
 // ControlStreamHandler registers connections with origintunneld and initiates graceful shutdown.
 type ControlStreamHandler interface {
-	ServeControlStream(ctx context.Context, rw io.ReadWriteCloser, connOptions *tunnelpogs.ConnectionOptions, shouldWaitForUnregister bool) error
+	// ServeControlStream handles the control plane of the transport in the current goroutine calling this
+	ServeControlStream(ctx context.Context, rw io.ReadWriteCloser, connOptions *tunnelpogs.ConnectionOptions) error
+	// IsStopped tells whether the method above has finished
 	IsStopped() bool
 }
 
@@ -61,7 +63,6 @@ func (c *controlStream) ServeControlStream(
 	ctx context.Context,
 	rw io.ReadWriteCloser,
 	connOptions *tunnelpogs.ConnectionOptions,
-	shouldWaitForUnregister bool,
 ) error {
 	rpcClient := c.newRPCClientFunc(ctx, rw, c.observer.log)
 
@@ -71,12 +72,7 @@ func (c *controlStream) ServeControlStream(
 	}
 	c.connectedFuse.Connected()
 
-	if shouldWaitForUnregister {
-		c.waitForUnregister(ctx, rpcClient)
-	} else {
-		go c.waitForUnregister(ctx, rpcClient)
-	}
-
+	c.waitForUnregister(ctx, rpcClient)
 	return nil
 }