User deletion complexities

[crspybits]

User deletion: If no other references to the sharing group, can we delete the sharing group? Put a comment to this effect in the user deletion. Probably defer this until we know for certain that we have foreign key references to all uses, to make testing better.

We also have an apparent race condition in integrity. In marking all files of the user as deleted, and other users trying to get a file index. Also seems like we need to do this marking of files as deleting using the usual system mechanisms of incrementing master version. Just don't delete the files from cloud storage.