vendor: Move back to upstream KCP (fixes #4407)

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4614

vendor/github.com/xtaci/kcp-go/rand.go

@@ -0,0 +1,25 @@
+package kcp
+
+import (
+	"crypto/md5"
+	"crypto/rand"
+	"io"
+)
+
+// nonceMD5 is a nonce generator for each packet header
+// which took the advantages of both MD5 and CSPRNG(like /dev/urandom).
+// The benchmark shows it's faster than previous CSPRNG only method.
+type nonceMD5 struct {
+	data [md5.Size]byte
+}
+
+// Nonce fills a nonce into the provided slice with no more than md5.Size bytes
+// the entropy will be updated whenever a leading 0 appears
+func (n *nonceMD5) Fill(nonce []byte) {
+	if n.data[0] == 0 { // 1/256 chance for entropy update
+		io.ReadFull(rand.Reader, n.data[:])
+	}
+	n.data = md5.Sum(n.data[:])
+	copy(nonce, n.data[:])
+	return
+}