Cache result of checking whether axis should be inverted for wms layers

This greatly improves speed of connecting to wms layers when a wms server has many available layers. Currently, when I try and connect to a wms which contains ~1000 layers QGIS hangs for a couple of minutes.   This hang occurs both when connecting via the add wms layer dialog and after actually adding a wms layer to the map. With this change there's only a couple of seconds wait for connecting/adding layers.
I've also commented out some debug messages which were flooding the log.