docs: @Nullable
      public TransactionConfidence get(Sha256Hash hash) {
          lock.lock();
          try {
              WeakConfidenceReference ref = table.get(hash);
              if (ref == null)
                  return null;
              TransactionConfidence confidence = ref.get();
              if (confidence != null)
                  return confidence;
              else
                  return null;
          } finally {
              lock.unlock();
          }
      }