Cache-only mode: various layers don't support it correctly

[gwaldron]

Describe the bug
Some layers, like coveragelayer, lifemaplayer, and in general layer types that reference other layers with LayerReference, tend to have problems with cache-only mode.

Expected behavior
Cache-only mode should always go to the highest-level cache and never bother trying to open dependent layers.

A refactor of the caching system in general MAY be in order