Verify deletion of file/objects when requested

[rudolphpienaar]

Some reports indicate that CUBE does not delete objects from storage. This needs to be checked/verified/fixed.

[jbernal0019]

• A PR was merged to make sure leftover size zero files are deleted if an exception happens when creating a file: Make is_staff user attribute read-only and delete leftover file in storage when exception is raised #641
• A PR was merged to make delete requests to pfcon more robust and add periodic task to retry failed requests: Make delete requests to pfcon more robust and add periodic task to retry failed requests #633

[jbernal0019]

A new PR was merged that should further help with solving this issue: Make async tasks for plugin instances more resilient to failures #648. This PR improves the resilience of async tasks related to a plugin instance to failures such as a task crash (e.g. because an uncaught exception) or worker crash (e.g. because of a SIGTERM signal). Currently the cleanup code just schedules a new request on the async queue to cancel the plugin instance.