This adds 'repozo --verify' to check your backup integrity

This pull request implements a new repozo mode `repozo -V` (or `--verify`) that checks the backup repository integrity by comparing file sizes and checksums against the `.dat` file.

It comes with tests for the newly-added code.

I've found it invaluable dealing with a complicated gpg-encrypted backup set of a 18-gig Data.fs where hitting Ctrl-C at the wrong moment during gpg decryption can leave a truncated .deltafs file, which then causes silent data corruption during `repozo -R` (see https://bugs.launchpad.net/zodb/+bug/906057).