- We always recalculate the reference count of a block before deleting
it locally, to make sure that it is indeed zero.
- If we had to fetch a remote block but we were not able to get it,
check that refcount is indeed > 0.
- Repair procedure that checks everything
- Fix bucket delete
- fix merge of bucket creation date
- Replace deletable with option in aliases
Rationale: if two aliases point to conflicting bucket, resolving
by making an arbitrary choice risks making data accessible when it
shouldn't be. We'd rather resolve to deleting the alias until
someone puts it back.