Replace the usages of KVState.keys() methods

[Neeharika-Sompalli]

Once MegaMap is implemented , in addition to stopping the use of KVState.size() method, using KVState.keys() is not going to be performance efficient.

    public Iterator<K> keys() {
        return iterateFromDataSource();
    }

For example, This is used to iterate through StakingInfo map. Replace the usage of keys method and instead get all nodes from roster entries.