Change payment notification listener hook name

[jdeeburke]

SV_WC_Payment_Gateway_Hosted automatically registers an action to listen for hosted transaction responses based on the value of get_class() -- see it here.

This has been fine in the past, but as we begin to move towards using namespaces in more plugins, this will become a problem as get_class() will result in the fully-qualified class name, including backslashes, therefore creating an action like woocommerce_api_skyverge\woocommerce\plugin_name\gateway which is pretty nasty-looking and won't work.

Instead, perhaps we should use the Gateway ID in the action rather than the class name.