code: public String remoteCheckpointDocID() {
          if (remoteCheckpointDocID != null) {
              return remoteCheckpointDocID;
          } else {
              if (db == null || !db.isOpen())
                  return null;
              return remoteCheckpointDocID(db.privateUUID());
          }
      }
docs: This is the _local document ID stored on the remote server to keep track of state.
  Its ID is based on the local database ID (the private one, to make the result unguessable)
  and the remote database's URL.
  
  @exclude