Get/Set last harvested time for a given harvest source

[johnbywater]

Implies that last harvested time must be stored on source by harvesting job (see #802).

[johnbywater]

Referencing ticket #728 has changed sprint.

[johnbywater]

Referencing ticket #799 has changed sprint.

[thejimmyg]

Perhaps this should be implemented differently so that each harvest attempt creates its own database entry with a timestamp and a status attached. We'll need to move to this when we move to a queue based system.

[thejimmyg]

I don't see why we'd really need to do this. Under the current system a job would be created just before the harvesting would be run so the creation date of the job is (give or take a few seconds) the date of the harvesting. If we moved to a queue system this would be done better anyway.

[thejimmyg]

We do have a requirement for this now. The job model has changed so that it is hidden from the user. We therefore want to know the timestamp the job started and the timestamp it finished. We'll therefore need migrations adding too.

[thejimmyg]

This will now be solved as part of the larger harvesting refactor. See #921.