p2p: fix discovery shutdown (#8725) (#8735)

Problem:
Some goroutines are blocked on shutdown:
1. table close <-tab.closed // because table loop pending
1. table loop <-refreshDone // because lookup shutdown blocks doRefresh
1. lookup shutdown <-it.replyCh // because it.queryfunc (findnode -
ensureBond) is blocked, and not returning errClosed (if it returns and
pushes to it.replyCh, then shutdown() will unblock)
1. findnode - ensureBond <-rm.errc // because the related replyMatcher
was added after loop() exited, so there's nothing to push errClosed and
unlock it

If addReplyMatcher channel is buffered, it is possible that
UDPv4.pending() adds a new reply matcher after closeCtx.Done().
Such reply matcher's errc result channel will never be updated, because
the UDPv4.loop() has exited at this point. Subsequent discovery
operations will deadlock.

Solution:
Revert to an unbuffered channel.

p2p/discover/v4_udp.go

@@ -160,7 +160,7 @@ func ListenV4(ctx context.Context, protocol string, c UDPConn, ln *enode.LocalNo
 		localNode:           ln,
 		db:                  ln.Database(),
 		gotreply:            make(chan reply, 10),
-		addReplyMatcher:     make(chan *replyMatcher, 10),
+		addReplyMatcher:     make(chan *replyMatcher),
 		gotkey:              make(chan v4wire.Pubkey, 10),
 		gotnodes:            make(chan nodes, 10),
 		replyTimeout:        cfg.ReplyTimeout,