Add Hippo Repository event listener support to notifying repository instance usages to be notified when a repository instance is closed

Currently HippoRepository instance usages have no means to detect if a the instance is still active or possibly shutdown.
This for instance breaks the HST repository connection pool in such circumstances, see issue HSTTWO-2567.

The solution is to enhance the HippoRepository API with a boolean method to allow checking if an instance is still alive, as well as register an event listener to be notified when it is closed.
Maybe this should be wired into a HippoServiceRegistry whiteboard pattern as well.