Create a checker to verify that shoots have been fully transferred

[paul-butcher]

Currently, the checks for successful transfers are designed to facilitate retrying failed transfers.

There is an assumption that the presence of at least part of a shoot in born-digital-accessions implies that the shoot has been transferred. It also checks the storage_ingests index to see if the files have been recorded as successfully ingested.

It does not check the files on the target to see if the whole shoot has been successfully transferred. It is possible that a large shoot would be reported as having been successfully ingested, even if it has only been partly ingested.

e.g. if a 7 part shoot has only transferred 5 parts to the target system, and all of them have been successfully ingested, then that shoot would erroneously be declared successful.

[paul-butcher]

This checker would need to iterate over all the object summaries in a shoot, and look for the corresponding files in the storage service. It would probably have to share the code for cracking apart large shoots by size, so that it can add the appropriate 00x