Clone associated relations when cloning ProtoParticle objects

This is a hack which simplifies the truth-matching persistence logic in Moore. By having the ProtoParticle cloner know about ProtoParticle-to-MCParticle relations, we don't need a separate algorithm which clones the relations. This is nice because the ProtoParticle cloner will only clone the subset of protos references by candidates, and also the relevant associations.

This will be used in an upcoming Moore MR. It's very far away from a clean solution, but I decided to leave that for a later clean-up of how microDST-like writing is done. If that never happens, the stuff here might still be useful, but should have improved test coverage (i.e. something beyond the Moore tests).