Carries out completion procedures after the
ImportProcessor or the
FailedImportHandler have finished their work.
These include notifying the user of the import outcome and cleaning up
allocated resources, e.g. scheduling deletion of import logs.
It consumes a processed import that was fetched from the import GC queue and
returns
Repeat if the finalisation procedure
failed because of a transient error condition and it should be retried or
Stop if finalisation should not be retried either
because it succeeded or because it failed but it's not possible to recover.