message: check revision for overflow
diff: @@ -79,8 +79,10 @@ where
  panic!("increment_revision invoked during a query computation");
  }
  + let old_revision = self.shared_state.revision.fetch_add(1, Ordering::SeqCst);
  + assert!(old_revision != usize::max_value());
  let result = Revision {
  - generation: 1 + self.shared_state.revision.fetch_add(1, Ordering::SeqCst) as u64,
  + generation: 1 + old_revision as u64,
  };
  debug!("increment_revision: incremented to {:?}", result);
  