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/manifest

@@ -17,6 +17,14 @@
 			"branch": "master",
 			"notests": true
 		},
+		{
+			"importpath": "github.com/AudriusButkevicius/kcp-go",
+			"repository": "https://github.com/AudriusButkevicius/kcp-go",
+			"vcs": "git",
+			"revision": "0ccc04f3b8a7bdf53e2d4d6d0769adbc7cb3851a",
+			"branch": "master",
+			"notests": true
+		},
 		{
 			"importpath": "github.com/AudriusButkevicius/pfilter",
 			"repository": "https://github.com/AudriusButkevicius/pfilter",
@@ -378,6 +386,14 @@
 			"path": "/leveldb",
 			"notests": true
 		},
+		{
+			"importpath": "github.com/templexxx/xor",
+			"repository": "https://github.com/templexxx/xor",
+			"vcs": "git",
+			"revision": "42f9c041c330b560afb991153bf183c25444bcdc",
+			"branch": "master",
+			"notests": true
+		},
 		{
 			"importpath": "github.com/thejerf/suture",
 			"repository": "https://github.com/thejerf/suture",
@@ -413,14 +429,6 @@
 			"path": "/qr",
 			"notests": true
 		},
-		{
-			"importpath": "github.com/xtaci/kcp-go",
-			"repository": "https://github.com/xtaci/kcp-go",
-			"vcs": "git",
-			"revision": "0b0731ef3f184a8985edcb4ca26a4b0598c6dc1a",
-			"branch": "master",
-			"notests": true
-		},
 		{
 			"importpath": "github.com/xtaci/smux",
 			"repository": "https://github.com/xtaci/smux",