MXS-1396: Fix persistent connection hangs

When a COM_CHANGE_USER was executed, it is possible that the server
responds with a AuthSwitchRequest packet instead of an OK packet. In this
case, the server sends a new scramble which must be used to create the 20
byte hash that is expected as the response.

include/maxscale/protocol/mysql.h

@@ -425,6 +425,9 @@ bool gw_read_backend_handshake(DCB *dcb, GWBUF *buffer);
 /** Send the server handshake response packet to the backend server */
 mxs_auth_state_t gw_send_backend_auth(DCB *dcb);
 
+/** Sends a response for an AuthSwitchRequest to the default auth plugin */
+int send_mysql_native_password_response(DCB* dcb);
+
 /** Write an OK packet to a DCB */
 int mxs_mysql_send_ok(DCB *dcb, int sequence, uint8_t affected_rows, const char* message);