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.go

@@ -0,0 +1,49 @@
+package xor
+
+// SIMD Extensions
+const (
+	none = iota
+	avx2
+	// first introduced by Intel with the initial version of the Pentium 4 in 2001
+	// so I think we can assume all amd64 has sse2
+	sse2
+)
+
+var extension = none
+
+// Bytes : chose the shortest one as xor size
+// it's better to use it for big data ( > 64bytes )
+func Bytes(dst, src0, src1 []byte) {
+	size := len(dst)
+	if size > len(src0) {
+		size = len(src0)
+	}
+	if size > len(src1) {
+		size = len(src1)
+	}
+	xorBytes(dst, src0, src1, size)
+}
+
+// BytesSameLen : all slice's length must be equal
+// cut size branch, save time for small data
+func BytesSameLen(dst, src0, src1 []byte) {
+	xorSrc1(dst, src0, src1)
+}
+
+// BytesSrc0 : src1 >= src0, dst >= src0
+// xor src0's len bytes
+func BytesSrc0(dst, src0, src1 []byte) {
+	xorSrc0(dst, src0, src1)
+}
+
+// BytesSrc1 : src0 >= src1, dst >= src1
+// xor src1's len bytes
+func BytesSrc1(dst, src0, src1 []byte) {
+	xorSrc1(dst, src0, src1)
+}
+
+// Matrix : all slice's length must be equal && != 0
+// len(src) must >= 2
+func Matrix(dst []byte, src [][]byte) {
+	xorMatrix(dst, src)
+}