Allow cache entries to be prioritized

## Use Case

If prefetched zarr chunks are inserted at the beginning of `VolumeCache`'s LRU queue, they may be evicted after data that has actually been (or is being) used. This could cause all sorts of (mildly) undesirable effects.

## Solution

Allow cache entries to be inserted with lower priority if they are the result of a prefetch. Resolve #152 before beginning work on this.