network: rework shutdown sequence of Server

Close transport and disconnect peers right in the Shutdown(), so that no new
connections would be accepted and so that all the peers would be disconnected
correctly (avoiding the same deadlock as in e2116e4).

pkg/network/server.go

@@ -183,8 +183,12 @@ func (s *Server) Start(errChan chan error) {
 // Shutdown disconnects all peers and stops listening.
 func (s *Server) Shutdown() {
 	s.log.Info("shutting down server", zap.Int("peers", s.PeerCount()))
-	s.bQueue.discard()
+	s.transport.Close()
 	s.discovery.Close()
+	for p := range s.peers {
+		p.Disconnect(errServerShutdown)
+	}
+	s.bQueue.discard()
 	close(s.quit)
 }
 
@@ -212,10 +216,6 @@ func (s *Server) run() {
 		}
 		select {
 		case <-s.quit:
-			s.transport.Close()
-			for p := range s.peers {
-				p.Disconnect(errServerShutdown)
-			}
 			return
 		case p := <-s.register:
 			s.lock.Lock()