protected int register() {
    processorId = keeper.put(this);
    return processorId;
  }
Construct new ReplyProcessor21.

@param dm the DistributionManager to use for messaging and membership
@param system the DistributedSystem connection
@param initMembers the collection of members this processor wants replies from
@param cancelCriterion optional CancelCriterion to use; will use the dm if null
geode-core/src/main/java/org/apache/geode/distributed/internal/ReplyProcessor21.java