lib/connections: Use our own fork of kcp (fixes #4063)

This updates kcp and uses our own fork which:

1. Keys sessions not just by remote address, but by remote address +
conversation id 2. Allows not to close connections that were passed directly
to the library. 3. Resets cache key if the session gets terminated.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4339
LGTM: calmh

vendor/github.com/xtaci/kcp-go/updater.go

@@ -1,107 +0,0 @@
-package kcp
-
-import (
-	"container/heap"
-	"sync"
-	"time"
-)
-
-var updater updateHeap
-
-func init() {
-	updater.init()
-	go updater.updateTask()
-}
-
-// entry contains a session update info
-type entry struct {
-	sid uint32
-	ts  time.Time
-	s   *UDPSession
-}
-
-// a global heap managed kcp.flush() caller
-type updateHeap struct {
-	entries  []entry
-	indices  map[uint32]int
-	mu       sync.Mutex
-	chWakeUp chan struct{}
-}
-
-func (h *updateHeap) Len() int           { return len(h.entries) }
-func (h *updateHeap) Less(i, j int) bool { return h.entries[i].ts.Before(h.entries[j].ts) }
-func (h *updateHeap) Swap(i, j int) {
-	h.entries[i], h.entries[j] = h.entries[j], h.entries[i]
-	h.indices[h.entries[i].sid] = i
-	h.indices[h.entries[j].sid] = j
-}
-
-func (h *updateHeap) Push(x interface{}) {
-	h.entries = append(h.entries, x.(entry))
-	n := len(h.entries)
-	h.indices[h.entries[n-1].sid] = n - 1
-}
-
-func (h *updateHeap) Pop() interface{} {
-	n := len(h.entries)
-	x := h.entries[n-1]
-	h.entries[n-1] = entry{} // manual set nil for GC
-	h.entries = h.entries[0 : n-1]
-	delete(h.indices, x.sid)
-	return x
-}
-
-func (h *updateHeap) init() {
-	h.indices = make(map[uint32]int)
-	h.chWakeUp = make(chan struct{}, 1)
-}
-
-func (h *updateHeap) addSession(s *UDPSession) {
-	h.mu.Lock()
-	heap.Push(h, entry{s.sid, time.Now(), s})
-	h.mu.Unlock()
-	h.wakeup()
-}
-
-func (h *updateHeap) removeSession(s *UDPSession) {
-	h.mu.Lock()
-	if idx, ok := h.indices[s.sid]; ok {
-		heap.Remove(h, idx)
-	}
-	h.mu.Unlock()
-}
-
-func (h *updateHeap) wakeup() {
-	select {
-	case h.chWakeUp <- struct{}{}:
-	default:
-	}
-}
-
-func (h *updateHeap) updateTask() {
-	var timer <-chan time.Time
-	for {
-		select {
-		case <-timer:
-		case <-h.chWakeUp:
-		}
-
-		h.mu.Lock()
-		hlen := h.Len()
-		now := time.Now()
-		for i := 0; i < hlen; i++ {
-			entry := heap.Pop(h).(entry)
-			if now.After(entry.ts) {
-				entry.ts = now.Add(entry.s.update())
-				heap.Push(h, entry)
-			} else {
-				heap.Push(h, entry)
-				break
-			}
-		}
-		if h.Len() > 0 {
-			timer = time.After(h.entries[0].ts.Sub(now))
-		}
-		h.mu.Unlock()
-	}
-}