@Override
  public int hashCode() {
    return (uuid != null ? uuid : uuidStr).hashCode();
  }
This gets a "version comparison group" to be used in the {@link #compareTo} method. If two blob IDs are in
different groups, they cannot be deemed equal to each other. This allows for comparison strategies that rely
on version-specific features or fields.
@return the "version comparison group" number.
ambry-commons/src/main/java/com/github/ambry/commons/BlobId.java