Change archive_date meaning in backup tasks to note record deletion date.

The write_s3 jobs now set s3_key/archive_sha only when a S3 upload was
successful. Before it always happened, as the `cmb` object was already
part of the session and got its changes committed.

The deletion jobs now pick work based on the archive_date, taking
all blocks with s3_key/archive_sha set, but without a archive_date.
After successful record deletion it sets the archive_date of the block
to avoid trying to delete the same records again.
Change `cmb` variable into `block`.

The c from cell measure block (cmb) got removed earlier.
Add limit argument to backup deletion tasks.

Also remove unused cleanup_zip argument.