`kgtk query` Process the graph cache without accidentally loading a file.

Enhance kgtk query to process graphs in the graph cache without running the risk of accidentally reloading them with an operating system file.

Create a new option, say --cache-only.

Plumb it through to sqlstore.py:has_graph(...)

The safety issue is addressed in commit 80a2f4ff along with a few other small issues with potentially incorrect aliases. So now a spurious file that matches an existing alias will not wipe out the cache for that graph by accident. Allowing locking of individual inputs or graphs or the whole cache DB to prevent modification might be a useful enhancement down the road, so I'll leave this issue open for now.

The branch issue/460/kypher-force-reuse has my strawperson changes.