Allow cache entries to be prioritized

[frasercl]

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.

[frasercl]

The problem here, more specifically: when data is loaded, the actual data required for display is fetched first, then we begin prefetching outward in time/space (T-1, T+1, T-2, T+2, etc.). The new data also gets inserted into the cache in this order... making it look as though deep prefetched data is most recently used and highest priority!