Feature: add snapshot size to snapshot metadata.

While the snapshot size (as a snapshot) on disk may not have much relevance because of the additive nature of snapshots (segments may already exist in the repository, so data transferred may be less than the total, and deleting a snapshot may not release all of the disk space because some segments may be retained for other snapshots...), it should be easy to capture the total size of the snapshot as it will require disk space to be restored, and this is an information which can be very useful when trying to determine before executing a restore, whether there is enough disk space on the target cluster.