[block-ref-repair] Block refcount recalculation and repair

- 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

src/util/error.rs

@@ -70,6 +70,9 @@ pub enum Error {
 	#[error(display = "Corrupt data: does not match hash {:?}", _0)]
 	CorruptData(Hash),
 
+	#[error(display = "Missing block {:?}: no node returned a valid block", _0)]
+	MissingBlock(Hash),
+
 	#[error(display = "{}", _0)]
 	Message(String),
 }