Finish remainder of the crawl workflow (SIPs and submission)

[anjackson]

• Mint ARKs for each payload file, incrementally.
• Store full WARC-to-ARK identifier mapping somewhere and update it over time.
• Build current METS SIP for final package (avoiding hard-coded fields - see below)
• Submit final SIP to DLS.
• Verify SIP appears in DLS.

There are a number of hard-coded fields in the SIP generation process. The ones in creator.py refer only to the temporary BagIt that is used to send the SIP to DLS so that's probably acceptable. However, the others (in mets.py) become part of the METS in the archival package.

Note that we could get the ClamD version string by posting the string VERSION to it's API.

Other fields should probably get picked up from the general configuration file, but note that this does not seem to be picked up correctly outside of the Celery run-time (TBC).