immediately remove sessions that were closed remotely

We only need to keep sessions that were closed locally, in order to
retransmit the CONNECTION_CLOSE packet. For sessions that were closed by
the peer, we don't need to wait for any more packets, and there's also
no CONNECTION_CLOSE to retransmit. The same applies for sessions that
were destroyed when receiving a Version Negotiation or a Retry packet.

packet_handler_map.go

@@ -51,6 +51,12 @@ func (h *packetHandlerMap) Add(id protocol.ConnectionID, handler packetHandler)
 	h.mutex.Unlock()
 }
 
+func (h *packetHandlerMap) Remove(id protocol.ConnectionID) {
+	h.mutex.Lock()
+	delete(h.handlers, string(id))
+	h.mutex.Unlock()
+}
+
 func (h *packetHandlerMap) Retire(id protocol.ConnectionID) {
 	h.retireByConnectionIDAsString(string(id))
 }