PSync

How Consumer::HelloData works:

 for /prefix/seq in in helloData:
    if subscribed to prefix and behind on sequence number:
        add to updates
    add /prefix/seq to available subscriptions

 tell application about available subscriptions [onHelloData callback]
 tell application about updates [onSyncData callback]

This is so that app when app renews hello interest, Consumer can tell the app only if it has missed some updates while sleeping.
But what is happening here is that Consumer receives the /prefix/seq as part of first hello data and then the app subscribes to it. So consumer has received hello data with the most up to date sequence number correctly. But there is no callback to the app because it was not subscribed to it.
And when the app does call addSubscription, the Consumer class inserts the prefix with a sequence number of zero and not the original sequence number that was received in HelloData. Seems like we would have to modify onHelloData callback to add the sequence number so that app knows about this latest sequence number and can give it to addSubscription.

https://gerrit.named-data.net/c/PSync/+/6183

HelloData cb is changed to return a prefix->seqNo map instead of list of prefixes.