use the new qtls SetWriteKey and SetReadKey interface

internal/handshake/qtls.go

@@ -1,8 +1,6 @@
 package handshake
 
 import (
-	"crypto"
-	"crypto/cipher"
 	"crypto/tls"
 	"net"
 	"time"
@@ -11,13 +9,6 @@ import (
 	"github.com/marten-seemann/qtls"
 )
 
-type cipherSuite interface {
-	Hash() crypto.Hash
-	KeyLen() int
-	IVLen() int
-	AEAD(key, nonce []byte) cipher.AEAD
-}
-
 type conn struct {
 	remoteAddr net.Addr
 }