Member hydration should use a members query instead of querying for every resource

[tpendragon]

Reasoning

Right now if a folder has 600 pages, it hydrates by sending 600 postgres queries. We can make that 1.

Acceptance Criteria

• Getting members for hydration takes only 1 query.

First step

Look at https://github.com/pulibrary/figgy/blob/ex_figgy/ex_manifest/apps/ex_manifest_api/lib/ex_manifest_api/resource/query.ex#L6-L21 for inspiration, where we gathered up members for an old Elixir-based manifest generation service.