Only update DNSLink when content is available on gateway

[icidasset]

Problem

Otherwise data may get lost.
See oddsdk/ts-odd#77 for more context.

Solution

Wait for content to be available and then update DNSLink.
Also report progress to API consumer.

Related to #312

[expede]

Not a bug per se — more a question of tradeoffs, since an online peer with those changes can serve them instantly.

The idea is that by making this change the server will behave more like a traditional uploader, and only provide the ID to others when it has a copy of the data. We can additionally broadcast the new Merkle root over IPNS pubsub (#363). This gives us the best of both: new pulls get the last known-good state. Active/online users get alerted that something has changed.

[icidasset]

We already have this right?