[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/block/lib.rs

@@ -11,3 +11,4 @@ mod metrics;
 mod rc;
 
 pub use block::zstd_encode;
+pub use rc::CalculateRefcount;