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/templexxx/xor/xor_other.go

@@ -0,0 +1,19 @@
+// +build !amd64 noasm
+
+package xor
+
+func xorBytes(dst, src0, src1 []byte, size int) {
+	bytesNoSIMD(dst, src0, src1, size)
+}
+
+func xorMatrix(dst []byte, src [][]byte) {
+	matrixNoSIMD(dst, src)
+}
+
+func xorSrc0(dst, src0, src1 []byte) {
+	bytesNoSIMD(dst, src0, src1, len(src0))
+}
+
+func xorSrc1(dst, src0, src1 []byte) {
+	bytesNoSIMD(dst, src0, src1, len(src1))
+}