[document_database] store metadata fields in a list object for easy reference

[signebedi]

I think we can initialize the metadata fields in their own method and then assign these on return to a self.metadata_fields object, which we can then use for quick checks.

[signebedi]

What we COULD do as a result of this change is insist that none of the field names are the same as any of these fields. This would in theory allow us to reconstruct document data in a less hierarchical way. But, I think that keeping the "data" and "metadata" subunits of a document is the superior method that is just a little more verbose and slightly (but not impossibly) harder to unpack... besides, signatures work perfectly when we are just signing the "data" and allowing the metadata to change....