Allow 2D XY slices to progressively refine

[ShrimpCryptid]

Use Case

• Load lower-resolution slice initially, then load in higher-resolution slice and swap.

Solution

• How do we orchestrate the sequence of requests?
• How is this integrated with prefetching? (prefetch at single fixed resolution)
• Add an option to stagger a sequence of requests to the pre-fetching API (RequestQueue).
  • Can be used for pre-fetching future frames without slowdowns when large amounts of data arrive at once, and can be interrupted by higher priority requests.

[toloudis]

also see #91 for ideas on how this could be expected to scale in the future