avoid lock contention when accepting new connections (#4313)

* avoid lock contention when accepting new connections

The server used to hold the packet handler map's lock while creating the
connection struct for a newly accepted connection. This was intended to
make sure that no two connections with the same Destination Connection
ID could be created.

This is a corner case: it can only happen if two Initial packets with
the same Destination Connection ID are received at the same time. If
the second one is received after the first one has already been
processed, it would be routed to the first connection. We don't need to
optimized for this corner case. It's ok to create a new connection in
that case, and immediately close it if this collision is detected.

* only pass 0-RTT to the connection if it was actually accepted

packet_handler_map.go

@@ -129,7 +129,7 @@ func (h *packetHandlerMap) Add(id protocol.ConnectionID, handler packetHandler)
 	return true
 }
 
-func (h *packetHandlerMap) AddWithConnID(clientDestConnID, newConnID protocol.ConnectionID, fn func() (packetHandler, bool)) bool {
+func (h *packetHandlerMap) AddWithConnID(clientDestConnID, newConnID protocol.ConnectionID, handler packetHandler) bool {
 	h.mutex.Lock()
 	defer h.mutex.Unlock()
 
@@ -137,12 +137,8 @@ func (h *packetHandlerMap) AddWithConnID(clientDestConnID, newConnID protocol.Co
 		h.logger.Debugf("Not adding connection ID %s for a new connection, as it already exists.", clientDestConnID)
 		return false
 	}
-	conn, ok := fn()
-	if !ok {
-		return false
-	}
-	h.handlers[clientDestConnID] = conn
-	h.handlers[newConnID] = conn
+	h.handlers[clientDestConnID] = handler
+	h.handlers[newConnID] = handler
 	h.logger.Debugf("Adding connection IDs %s and %s for a new connection.", clientDestConnID, newConnID)
 	return true
 }