Unused resource clean-up (gc)

This framework will need some kind of cleanup for when resources haven't been used for a while. It would probably be good to make this with some sensible defaults, but overridable by an API.

Sensible defaults:
When a resource is no longer required by any currently mounted component for X seconds/minutes, it is removed from the resource map.

No files, tests, or entry points are named. Start by tracing the resource map and how mounted components indicate that resources are still required; define sensible cleanup defaults and an overridable API. Done means unused resources are removed after the configured interval without affecting resources required by mounted components.

Written by the indexing model from the issue text.