feat: block reference counting

This patch prevents deleting of a block from a vault if the block has
files referring to it. It adds a ConstraintError that is thrown if this
is attempted.
Storage last-modification-time in vault statistics
Merge pull request #117 from jc7998/vault_stats

Storage last-modification-time in vault statistics
Merge pull request #114 from painterjd/feat_no_delete_block_with_refs

feat: block reference counting
chore: misc formatting, comment fixes