Refactor Interactions to enable loading historical Interactions

[mnzaki]

agent.processJWT is now responsible for ~validating incoming JWTs,~ storing JWTs as necessary, and "encountering" new DIDs based on jwt.payload.pca Interaction.processInteractionToken need not concern itself with those, and can remain rather "pure"

This PR does not affect the public API's inputs/outputs, but does affect behavior: if someone was directly using Interaction.processInteractionToken then they should now also handle the storage ~and validation~ concerns

[mnzaki]

So I haven't really arrived at a good answer to "should we be validating at the layer of Interaction.processInteractionToken" and instead I just split out processInteractionToken into another private method _processToken which is used internally when we don't want to validate, but left processInteractionToken the same.... until further/more major refactoring of Interaction

Note that I still left the storage and PCA processing changes (i.e. moved them to agent.processJWT) because I think those make a lot more sense there, and it improves performance quite a bit for loading stuff from DB