update mint, and adapt to the new use of the extension handler

vendor/github.com/bifurcation/mint/dtls.go

@@ -1,7 +1,28 @@
 package mint
 
+import (
+	"fmt"
+)
+
 // This file is a placeholder. DTLS-specific stuff (timer management,
 // ACKs, retransmits, etc. will eventually go here.
 const (
 	initialMtu = 1200
 )
+
+func wireVersion(h *HandshakeLayer) uint16 {
+	if h.datagram {
+		return dtls12WireVersion
+	}
+	return tls12Version
+}
+
+func dtlsConvertVersion(version uint16) uint16 {
+	if version == tls12Version {
+		return dtls12WireVersion
+	}
+	if version == tls10Version {
+		return 0xfeff
+	}
+	panic(fmt.Sprintf("Internal error, unexpected version=%d", version))
+}