[DPM Optimization] Change List to HashSet in MulticastGoalStateV2

[xieus]

PR #713 changes List to HashSet for efficiency consideration (avoid duplication etc.)

We would like to apply similar changes to other Lists in
https://github.com/futurewei-cloud/alcor/pull/713DPM/src/main/java/com/futurewei/alcor/dataplane/entity/MulticastGoalStateV2.java.
For example,
private List<String> vpcIds; private List<String> topics; private List<String> subTopics;