Task #9351 #9353 #9355: let the cache be updated in the background only, yield cache results immediately in getDiskUsageForPath, even if they are old

Task #9351 #9353 #9355: added notifications on updates of diskusage for path. Handle notifications with datamanagementbuslistener (which was the cleanupbuslistener before).

Task #9351 #9353 #9355: added notifications on deletion of path/task. Handle notifications with cleanupbuslistener. Various fixes and improvements in cache

Task #9350: added cache to service for faster response times which is prepopulated on observation finished events, stored on disk, and refreshed every 15 min