Cluster listeners and XSite

As per info from Will and Pedro, when a cluster listener is defined in multiple sites, updates coming from another site might trigger listeners in a site.

This might not always be what you want it to happen, e.g. if a site is backup of another and the event should only be processed in the site of origin.

Tests should be added to xsite testsuite to verify this. This should be documented, as well as workarounds to avoid this happening (e.g. server side filter?)