[ARIES-992] Get an IllegalStateException in BundleEventHook.handleExplicitlyInstalledBundleBundleContext

This happens when explicitly installing or uninstalling bundles while the subsystems bundle is deactivating.
The bundle event hook must be blocked while the root subsystem is being stopped. The blocked events then proceed
once deactivation is complete which results in the exception since the activator instance is no longer valid.
Events are now queued while subsystems is shutting down, as they are during initialization. Once the shutdown is
complete and the event hook has been unregistered, any remaining events are processed prior to returning from the
BundleActivator.stop() method.

git-svn-id: https://svn.apache.org/repos/asf/aries/trunk@1428038 13f79535-47bb-0310-9956-ffa450edef68