Improve RSGroupInfo constructors

RSGroupInfo(String name, SortedSet<Address> servers, SortedSet<TableName> tables) {
    this.name = name;
    this.servers = servers == null? new TreeSet<>(): servers;
    this.servers.addAll(servers);
    this.tables = new TreeSet<>(tables);
}

2 improvements could be made:

• When servers is not null, addAll(servers) tries to add all items in servers again. Seems not needed
• new TreeSet<>(tables) has no null check on tables. The constructor of TreeSet does not do the null check either. It leads to NullPointerException if tables is null