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/AudriusButkevicius/kcp-go/xor.go

@@ -0,0 +1,110 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package kcp
+
+import (
+	"runtime"
+	"unsafe"
+)
+
+const wordSize = int(unsafe.Sizeof(uintptr(0)))
+const supportsUnaligned = runtime.GOARCH == "386" || runtime.GOARCH == "amd64" || runtime.GOARCH == "ppc64" || runtime.GOARCH == "ppc64le" || runtime.GOARCH == "s390x"
+
+// fastXORBytes xors in bulk. It only works on architectures that
+// support unaligned read/writes.
+func fastXORBytes(dst, a, b []byte) int {
+	n := len(a)
+	if len(b) < n {
+		n = len(b)
+	}
+
+	w := n / wordSize
+	if w > 0 {
+		wordBytes := w * wordSize
+		fastXORWords(dst[:wordBytes], a[:wordBytes], b[:wordBytes])
+	}
+
+	for i := (n - n%wordSize); i < n; i++ {
+		dst[i] = a[i] ^ b[i]
+	}
+
+	return n
+}
+
+func safeXORBytes(dst, a, b []byte) int {
+	n := len(a)
+	if len(b) < n {
+		n = len(b)
+	}
+	ex := n % 8
+	for i := 0; i < ex; i++ {
+		dst[i] = a[i] ^ b[i]
+	}
+
+	for i := ex; i < n; i += 8 {
+		_dst := dst[i : i+8]
+		_a := a[i : i+8]
+		_b := b[i : i+8]
+		_dst[0] = _a[0] ^ _b[0]
+		_dst[1] = _a[1] ^ _b[1]
+		_dst[2] = _a[2] ^ _b[2]
+		_dst[3] = _a[3] ^ _b[3]
+
+		_dst[4] = _a[4] ^ _b[4]
+		_dst[5] = _a[5] ^ _b[5]
+		_dst[6] = _a[6] ^ _b[6]
+		_dst[7] = _a[7] ^ _b[7]
+	}
+	return n
+}
+
+// xorBytes xors the bytes in a and b. The destination is assumed to have enough
+// space. Returns the number of bytes xor'd.
+func xorBytes(dst, a, b []byte) int {
+	if supportsUnaligned {
+		return fastXORBytes(dst, a, b)
+	}
+	// TODO(hanwen): if (dst, a, b) have common alignment
+	// we could still try fastXORBytes. It is not clear
+	// how often this happens, and it's only worth it if
+	// the block encryption itself is hardware
+	// accelerated.
+	return safeXORBytes(dst, a, b)
+}
+
+// fastXORWords XORs multiples of 4 or 8 bytes (depending on architecture.)
+// The arguments are assumed to be of equal length.
+func fastXORWords(dst, a, b []byte) {
+	dw := *(*[]uintptr)(unsafe.Pointer(&dst))
+	aw := *(*[]uintptr)(unsafe.Pointer(&a))
+	bw := *(*[]uintptr)(unsafe.Pointer(&b))
+	n := len(b) / wordSize
+	ex := n % 8
+	for i := 0; i < ex; i++ {
+		dw[i] = aw[i] ^ bw[i]
+	}
+
+	for i := ex; i < n; i += 8 {
+		_dw := dw[i : i+8]
+		_aw := aw[i : i+8]
+		_bw := bw[i : i+8]
+		_dw[0] = _aw[0] ^ _bw[0]
+		_dw[1] = _aw[1] ^ _bw[1]
+		_dw[2] = _aw[2] ^ _bw[2]
+		_dw[3] = _aw[3] ^ _bw[3]
+		_dw[4] = _aw[4] ^ _bw[4]
+		_dw[5] = _aw[5] ^ _bw[5]
+		_dw[6] = _aw[6] ^ _bw[6]
+		_dw[7] = _aw[7] ^ _bw[7]
+	}
+}
+
+func xorWords(dst, a, b []byte) {
+	if supportsUnaligned {
+		fastXORWords(dst, a, b)
+	} else {
+		safeXORBytes(dst, a, b)
+	}
+}