Trash purging causes errors in queues

[demiankatz]

Currently, when you purge the trash, it causes a high volume of error messages in the indexing and metadata queues.

This is because deleting a datastream triggers an object-level reindexing operation, and any change to the MASTER datastream triggers metadata regeneration.

We may need to introduce some special handling so that when a datastream is deleted, a 404 error during object reindexing is not treated as a fatal error.

[demiankatz]

Possible approach to address at least some of the errors: if an index operation receives a 404 error and there is a queued delete operation, do not treat the 404 as an error.