Persist OriginalRecord

This is a substantial rewrite of OriginalRecord to allow them to persist to LDP.
The thinking here is that `Krikri::LDP` represents a self-contained feature set, so the engine requires it, and autoload responsibilities are moved to the module.  I don't think it makes sense to do a similar thing with `IndexService`.

I expect that more LDP mixins (for BasicContainers, maybe?) will be added at some point.  It may be useful at some point to extract all of this into a generalized library (`ActiveTriples::LDP` or something like).