Design sketch for quota support at the IFilesystem layer

This notes where changes are necessary support some volume size features.  Maximum volume size is a good approximation of the idea of quotas without being overly backend specific (for example, EBS volumes can't have quotas, they just have a size).
`size` seems more like a piece of data here, to me.  Lots of tests exercise the implementation with less than the full range of valid data.  I think this test is probably okay as-is (or at least a better version of the test isn't obvious to me).