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_amd64.go

@@ -0,0 +1,118 @@
+package xor
+
+func init() {
+	getEXT()
+}
+
+func getEXT() {
+	if hasAVX2() {
+		extension = avx2
+	} else {
+		extension = sse2
+	}
+	return
+}
+
+func xorBytes(dst, src0, src1 []byte, size int) {
+	switch extension {
+	case avx2:
+		bytesAVX2(dst, src0, src1, size)
+	default:
+		bytesSSE2(dst, src0, src1, size)
+	}
+}
+
+// non-temporal hint store
+const nontmp = 8 * 1024
+const avx2loopsize = 128
+
+func bytesAVX2(dst, src0, src1 []byte, size int) {
+	if size < avx2loopsize {
+		bytesAVX2mini(dst, src0, src1, size)
+	} else if size >= avx2loopsize && size <= nontmp {
+		bytesAVX2small(dst, src0, src1, size)
+	} else {
+		bytesAVX2big(dst, src0, src1, size)
+	}
+}
+
+const sse2loopsize = 64
+
+func bytesSSE2(dst, src0, src1 []byte, size int) {
+	if size < sse2loopsize {
+		bytesSSE2mini(dst, src0, src1, size)
+	} else if size >= sse2loopsize && size <= nontmp {
+		bytesSSE2small(dst, src0, src1, size)
+	} else {
+		bytesSSE2big(dst, src0, src1, size)
+	}
+}
+
+func xorMatrix(dst []byte, src [][]byte) {
+	switch extension {
+	case avx2:
+		matrixAVX2(dst, src)
+	default:
+		matrixSSE2(dst, src)
+	}
+}
+
+func matrixAVX2(dst []byte, src [][]byte) {
+	size := len(dst)
+	if size > nontmp {
+		matrixAVX2big(dst, src)
+	} else {
+		matrixAVX2small(dst, src)
+	}
+}
+
+func matrixSSE2(dst []byte, src [][]byte) {
+	size := len(dst)
+	if size > nontmp {
+		matrixSSE2big(dst, src)
+	} else {
+		matrixSSE2small(dst, src)
+	}
+}
+
+//go:noescape
+func xorSrc0(dst, src0, src1 []byte)
+
+//go:noescape
+func xorSrc1(dst, src0, src1 []byte)
+
+//go:noescape
+func bytesAVX2mini(dst, src0, src1 []byte, size int)
+
+//go:noescape
+func bytesAVX2big(dst, src0, src1 []byte, size int)
+
+//go:noescape
+func bytesAVX2small(dst, src0, src1 []byte, size int)
+
+//go:noescape
+func bytesSSE2mini(dst, src0, src1 []byte, size int)
+
+//go:noescape
+func bytesSSE2small(dst, src0, src1 []byte, size int)
+
+//go:noescape
+func bytesSSE2big(dst, src0, src1 []byte, size int)
+
+//go:noescape
+func matrixAVX2small(dst []byte, src [][]byte)
+
+//go:noescape
+func matrixAVX2big(dst []byte, src [][]byte)
+
+//go:noescape
+func matrixSSE2small(dst []byte, src [][]byte)
+
+//go:noescape
+func matrixSSE2big(dst []byte, src [][]byte)
+
+//go:noescape
+func hasAVX2() bool
+
+//go:noescape
+func hasSSE2() bool