Fix potential p2p shutdown hangup (#10626)

This is a fix for: 

#10192

This fixes is a deadlock in v4_udp.go where 
* Thread A waits on mutex.Lock() in resetTimeout() called after reading
listUpdate channel.
* Thread B waits on listUpdate <- plist.PushBack(p) called after locking
mutex.Lock()
  
This fix decouples the list operations which need locking from the
channel operations which don't by storing the changes in local
variables. These updates are used for resetting a timeout - which is not
order dependent.

p2p/discover/v4_udp.go

@@ -606,14 +606,15 @@ func (t *UDPv4) loop() {
 			return
 
 		case p := <-t.addReplyMatcher:
-			func() {
-				mutex.Lock()
-				defer mutex.Unlock()
-				p.deadline = time.Now().Add(t.replyTimeout)
-				listUpdate <- plist.PushBack(p)
-			}()
+			mutex.Lock()
+			p.deadline = time.Now().Add(t.replyTimeout)
+			back := plist.PushBack(p)
+			mutex.Unlock()
+			listUpdate <- back
 
 		case r := <-t.gotreply:
+			var removals []*list.Element
+
 			func() {
 				mutex.Lock()
 				defer mutex.Unlock()
@@ -629,7 +630,7 @@ func (t *UDPv4) loop() {
 						if requestDone {
 							p.errc <- nil
 							plist.Remove(el)
-							listUpdate <- el
+							removals = append(removals, el)
 						}
 						// Reset the continuous timeout counter (time drift detection)
 						contTimeouts = 0
@@ -638,6 +639,10 @@ func (t *UDPv4) loop() {
 				r.matched <- matched
 			}()
 
+			for _, el := range removals {
+				listUpdate <- el
+			}
+
 		case key := <-t.gotkey:
 			go func() {
 				if key, err := v4wire.DecodePubkey(crypto.S256(), key); err == nil {