Add listener method for removal request

Normally we can't accurately implement a mode-change notification method on ServiceListener for mode change because mode changes are non-blocking and listener methods are asynchronous, thus the order that the listener methods are invoked may not accurately reflect the order of the modes that were set on the service.

However it is important to be able to know when a service remove is requested because some applications (i.e. AS7) need to be able to detect when the container is "at rest" versus "busy", in order to display a summary of deployment status.

Fortunately, REMOVE is a special mode in that once a service is set to REMOVE, it cannot be changed again, thus the aforementioned out-of-order problem cannot arise. Add a service listener method which notifies the listener that a removal has been requested.