WIP: basic approach for collecting triples with distributed cache

This breaks some unit tests which are commented out and it doesn't
yield the desired JSON-LD yet (it seems like blank nodes are not
handled correctly in Jena or jsonld-jena).

Also, this would not scale as implemented here - we would at least
need to use map files for the cached subjects to avoid keeping them
in RAM on every node.
Add unit test for JSON-LD conversion w/ blank nodes, fix job issue

This is still work in progress at this point. The JSON-LD output
for input with blank nodes has changed significantly and is not
compatible with the rest of the system.
Fix issue with removed unit tests and add them back