Provide IExternalGC support for use with zc.zodbdgc

[jamadden]

This requires implementing one method that deletes an individual object (revision? I've only done this in history-free schemas).

Right now this would have to be integrated, which means a lot of plumbing through the adapter layer, but zc.zodbdgc can be changed to use zope.interface to request an alternate implementation (adapter).

[jamadden]

Does anyone use multi-databases with history-preserving schemas? I suspect that implementing IExternalGC correctly is likely to be quite a bit more complicated there.

I only use history-free schemas, so that's the only case I'm personally concerned about.