Extract shared session information in LocalClient constructor

When the LocalClient is constructed, it is possible to extract all the
needed information at that time. The only obstacle is the fact that the
LocalClient is constructed at the same time the session is. Since the
client DCB is created before the session, it is safe to extract the shared
data directly from it.

include/maxscale/protocol/mysql.h

@@ -487,14 +487,14 @@ int gw_decode_mysql_server_handshake(MySQLProtocol *conn, uint8_t *payload);
 /**
  * Create a response to the server handshake
  *
- * @param session         Session object
+ * @param client          Shared session data
  * @param conn            MySQL Protocol object for this connection
  * @param with_ssl        Whether to create an SSL response or a normal response packet
  * @param ssl_established Set to true if the SSL response has been sent
  *
  * @return Generated response packet
  */
-GWBUF* gw_generate_auth_response(MXS_SESSION* session, MySQLProtocol *conn,
+GWBUF* gw_generate_auth_response(MYSQL_session* client, MySQLProtocol *conn,
                                  bool with_ssl, bool ssl_established);
 
 /** Read the backend server's handshake */