- affinity info is similar to locks, but with a completely different
lifetime.
- streamline the main loop of the engine, so that it can do two passes:
first pass shuns paths with the wrong affinity. If no good path is found,
those are considered during the second pass.
- make the Core factory aware of what hosts might be running, so that
affinity info for machines removed from a config file will be ignored.
thanks to landry@ for a few tests.