Make it possible to ask a RelationshipSet for all its names
Make deb_pkg_tools.utils.atomic_lock() blocking by default
Improve performance: Cache results of RelationshipSet.matches()
Implement proper package metadata cache using SQLite 3.x (high performance)

I've been working on CPU and disk I/O intensive package analysis
across hundreds of package archives which is very slow even on
my MacBook Air with four cores and an SSD. I decided to rip the
ad-hoc cache in scan_packages() out and refactor it into a more
general purpose persistent, multiprocess cache implemented on
top of SQLite 3.x.