private void addToBeginningOfPool(ReplicaId replicaId) {
    modifyReplicasInPoolOrInFlightCount(1);
    replicaPool.addFirst(replicaId);
  }
Add a replica to the beginning of the replica pool linked list.
@param replicaId the replica to add.
ambry-router/src/main/java/com/github/ambry/router/SimpleOperationTracker.java