net/dns{., resolver}: time out DNS queries after 10 seconds (#4690)

Fixes https://github.com/tailscale/corp/issues/5198

The upstream forwarder will block indefinitely on `udpconn.ReadFrom` if no
reply is recieved, due to the lack of deadline on the connection object.

There still isn't a deadline on the connection object, but the automatic closing
of the context on deadline expiry will close the connection via `closeOnCtxDone`,
unblocking the read and resulting in a normal teardown.

Signed-off-by: Tom DNetto <tom@tailscale.com>

net/dns/manager.go

@@ -331,6 +331,9 @@ func (m *Manager) NextPacket() ([]byte, error) {
 	return buf, nil
 }
 
+// Query executes a DNS query recieved from the given address. The query is
+// provided in bs as a wire-encoded DNS query without any transport header.
+// This method is called for requests arriving over UDP and TCP.
 func (m *Manager) Query(ctx context.Context, bs []byte, from netaddr.IPPort) ([]byte, error) {
 	select {
 	case <-m.ctx.Done():
@@ -460,7 +463,7 @@ func (m *Manager) HandleTCPConn(conn net.Conn, srcAddr netaddr.IPPort) {
 		responses:    make(chan []byte),
 		readClosing:  make(chan struct{}),
 	}
-	s.ctx, s.closeCtx = context.WithCancel(context.Background())
+	s.ctx, s.closeCtx = context.WithCancel(m.ctx)
 	go s.handleReads()
 	s.handleWrites()
 }