_artifactcache/ostreecache.py: Rework so that all remotes for a ref are tracked

This commit adds a couple of simple classes in order to reduce the
number of different dictionaries tracking the state of the remotes
within the OSTreeCache object.

It also extends the internal ref map to remember all the remotes that
store a given ref, not just the highest priority ref, and modifies the
remote_contains_key() method to expose that data.