Memory limited job runner

This is another component needed for the new splitfile code (db4o purge).
Good point. I'm okay with this concept. Two remaining issues come to mind:

1. If it's called "new splitfile code" it is unclear and will inevitably become "current" or even "old." It'd make more sense to me to refer to it some other way - maybe distinguishing characteristics of its operation.
2. Ideally the FIXME should be resolved or at least have more thought put into it before being merged.