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

lib/protocol/benchmark_test.go

@@ -8,8 +8,8 @@ import (
 	"net"
 	"testing"
 
+	"github.com/AudriusButkevicius/kcp-go"
 	"github.com/syncthing/syncthing/lib/dialer"
-	"github.com/xtaci/kcp-go"
 )
 
 func BenchmarkRequestsRawTCP(b *testing.B) {