Make defaultContext a node

[cap10morgan]

Closes #449

Depends on fluree/json-ld#18

A couple things I'm slightly unsure of:

1. In commit-data/add-commit-flakes when the default context's sid already exists, do we need to add that flake (line 506) or is that redundant?
2. Would default context flakes ever show up in retracts? I'm not filtering them out currently, but we should if they can. Probably depends on how the new defaultContext updating code works.

[zonotope]

This looks good to me. My guess is that the flake added for the defaultContext is redundant, but perhaps @bplatz could chime in on that and whether defaultContext flakes could appear in retracts

[zonotope]

does this flake get created even when there was another context (different from the default) supplied with the commit?

[cap10morgan]

It shouldn't since that's just a context, not a default context. I don't think it would given the different key naming.

[bplatz]

I think this is a good call where you have it.

I think we could go any way, including not storing it as a Flake at all. I think the balance is thinking if it will be useful in a query for people to ask questions.

I think the most important question will be when did the context change and what are the addresses... so having flakes to answer this I think would be useful:

{:select [?t ?ctx-id ?ctx-address]
 :where [[?t :f/defaultContext ?ctx-id]
               [?ctx-id :f/address ?ctx-address]]}

Which is exactly what you have.

You are also linking the ?ctx-id to every t. That might be a bit excessive, but I can see it being useful for asking for it at a specific t (e.g. t=42 here) with:

{:select [?ctx-id ?ctx-address]
 :where [[42 :f/defaultContext ?ctx-id]
               [?ctx-id :f/address ?ctx-address]]}

Even perhaps excessive, it is a relatively compact amount of information so if not needed, its not like we are paying a super high price.

So I like your decision here. I think we'll get better feedback once we have more users and can change course if necessary then.