lib/connections, vendor: Change KCP mux to SMUX

Closes #4032

vendor/github.com/xtaci/smux/frame.go

@@ -0,0 +1,60 @@
+package smux
+
+import (
+	"encoding/binary"
+	"fmt"
+)
+
+const (
+	version = 1
+)
+
+const ( // cmds
+	cmdSYN byte = iota // stream open
+	cmdFIN             // stream close, a.k.a EOF mark
+	cmdPSH             // data push
+	cmdNOP             // no operation
+)
+
+const (
+	sizeOfVer    = 1
+	sizeOfCmd    = 1
+	sizeOfLength = 2
+	sizeOfSid    = 4
+	headerSize   = sizeOfVer + sizeOfCmd + sizeOfSid + sizeOfLength
+)
+
+// Frame defines a packet from or to be multiplexed into a single connection
+type Frame struct {
+	ver  byte
+	cmd  byte
+	sid  uint32
+	data []byte
+}
+
+func newFrame(cmd byte, sid uint32) Frame {
+	return Frame{ver: version, cmd: cmd, sid: sid}
+}
+
+type rawHeader []byte
+
+func (h rawHeader) Version() byte {
+	return h[0]
+}
+
+func (h rawHeader) Cmd() byte {
+	return h[1]
+}
+
+func (h rawHeader) Length() uint16 {
+	return binary.LittleEndian.Uint16(h[2:])
+}
+
+func (h rawHeader) StreamID() uint32 {
+	return binary.LittleEndian.Uint32(h[4:])
+}
+
+func (h rawHeader) String() string {
+	return fmt.Sprintf("Version:%d Cmd:%d StreamID:%d Length:%d",
+		h.Version(), h.Cmd(), h.StreamID(), h.Length())
+}