Import Turbo Tunnel support code.

Copied and slightly modified from
https://gitweb.torproject.org/pluggable-transports/meek.git/log/?h=turbotunnel&id=7eb94209f857fc71c2155907b0462cc587fc76cc
https://github.com/net4people/bbs/issues/21

RedialPacketConn is adapted from clientPacketConn in
c64a61c6da/obfs4proxy/turbotunnel_client.go
https://github.com/net4people/bbs/issues/14#issuecomment-544747519

common/turbotunnel/clientid.go

@@ -0,0 +1,28 @@
+package turbotunnel
+
+import (
+	"crypto/rand"
+	"encoding/hex"
+)
+
+// ClientID is an abstract identifier that binds together all the communications
+// belonging to a single client session, even though those communications may
+// arrive from multiple IP addresses or over multiple lower-level connections.
+// It plays the same role that an (IP address, port number) tuple plays in a
+// net.UDPConn: it's the return address pertaining to a long-lived abstract
+// client session. The client attaches its ClientID to each of its
+// communications, enabling the server to disambiguate requests among its many
+// clients. ClientID implements the net.Addr interface.
+type ClientID [8]byte
+
+func NewClientID() ClientID {
+	var id ClientID
+	_, err := rand.Read(id[:])
+	if err != nil {
+		panic(err)
+	}
+	return id
+}
+
+func (id ClientID) Network() string { return "clientid" }
+func (id ClientID) String() string  { return hex.EncodeToString(id[:]) }