Improve offline IPNS (record) support

[Stebalien]

Libp2p should have better offline/disconnected-network record support:

• On resolve, IPNS records should be cached locally. This will:
  • Help with streaming resolve. Peers searching for popular records will get back "best effort" results along the way. We still won't be able to stop the search early as these "best effort" copies of the records won't be updated but it can help.
  • Help with offline situations. If a node resolves a record while online, they'll continue to have a copy of that record when offline. It may not be up-to-date but it's better than nothing.
  • Help with resolving on a disconnected network. If a subset of nodes "become" the entire DHT, they'll have all the records they've resolved recently.
• Users should be allowed to publish DHT records while offline (or in a disconnected network). DHT rebalancing (to be implemented) should take care of moving these records to the right place at the end of the day.

Dependencies:

• https://github.com/libp2p/go-libp2p-kad-dht/pull/291. Introducing this would break IPNS at the moment as we don't properly complete the query.

[aschmahmann]

Two notes:

1) PubSub Value Router (e.g. IPNS over PubSub) will handle offline record support (already in master, waiting on the next release) 2) The proposed DHT changes that have peers remove themselves from the network if they think they're undialable will break offline IPNS over DHT. Dealing with an offline DHT that is separate from a public network is a separate issue.

In my eyes 2 is the libp2p request/discussion. Either we want non-dialable nodes to be able to use their own DHT, or we have to tell people that they should utilize other protocols (e.g. MDNS + PubSub) when they're concerned about offline use.