code: public boolean seen(long msgId) {
      boolean seen = nonContiguousMsgIds.contains(msgId) || msgId <= highestContiguousMsgId;
      tryReconcile();
      return seen;
    }
docs: Check wheather the given message id is already seen by track call.
  
  @param msgId Message Identifier to be checked.
  @return true if the given msgId is already tracked otherwise false.