Add checksums to tarstore, and partially migrate to using it for storage.

Add checksums to tarstore, and partially migrate to using it for storage.

Each tarfile segment gets a file called "checksums" now which contains
SHA-1 hashes of all objects contained in it.  There is currently no
checksum computed for the checksum file itself.

Begin migrating scandir to using the tar-based storage for backups.
Backups in the old format are still written, and there is still some
other cleanup to be done, but most of the raw data is currently handled.