Implement various harvesting strategies properly.

[miku]

metha should implement various harvesting strategies:

• normal/default (for standard conform endpoints), harvest windows, daily, monthly, yearly, all
• single records, so individual records may fail or servers are not overloaded
• other modes: all at once

Implementation ideas:

Instead of relying only on files, introduce a small manifest.json describing the harvested content (ids, dates, harvesting dates, files).

[miku]

metha was meant be a very simple program (no database, only files and not even metadata about files). In order to keep it simple, a more resilient harvesting approach has been implemented in a separate program: oaicrawl.