S3 checksum storage

[MarcusEbert]

From the documentation that is currently online, it seems the checksum is not stored in the metadata of a S3 object so far.
Switching from Dynafed to xrootd, we also changed the dynafed-storage-stats tools, that did the above, to work with an xrootd installation.

If useful for others, feel free to use from https://github.com/hep-gc/uvic-heprc-ansible-playbooks/tree/master/roles/xrootd/files
the s3-storage-stats together with a script like
https://github.com/hep-gc/uvic-heprc-ansible-playbooks/blob/master/roles/xrootd/templates/checksum.sh