Figure out the codex query API

[m0ar]

While there are a mixed bag of query functions, we need to create a logical API for composedb requests. We can already resolve the underlying stream state under several constraints, but explorative querying isn't yet exported. Querying just the basic entity is simple, but traversing relations etc is a bit more tricky to reach logical semantics with.

[m0ar]

Also, some convention for paging responses. Currently thinking pointer and nextPage boolean, or returning a generator/observable with the tail request.