Allow different object processing version and sources processing version in `fastdb_loader.py` (and the things that use it)

[rknop]

Proper usage will be to make sure that there is only one base processing version used for objects for a given processing version.

Probably.

Whereas sources and forced sources can have multiple processing versions.

In any event, somebody who's loading the database really needs to know what he's doing, and needs the option for the object base processing version to be different from the sources base processing version.

[rknop]

See Issue #70