CASCache improvements

This branch extracts various CASCache improvements from !2095:

• Move _reachable_refs_dir() method from cascache.py to testutils
• Also fetch file blobs in _fetch_directory() (eliminate code duplication)
• Remove _ensure_blob() method
• Remove unused parameters from add_object()
• Add add_objects() method
• Optimize _fetch_tree() using add_objects() (reduce gRPC round-trips)
• Generalize remote_missing_blobs() into missing_blobs()
• Add open() method