Offline Message Queue

There's an open problem we'd like to resolve:

1. User A generates some content and adds it to IPFS. They want to deliver this content to user B.
2. User A tries to connect to user B. However, it turns user B is offline.
3. User A goes offline.

Currently, we have no good way to deliver this information to user B. Textile uses the DHT as follows:

1. User A fails to connect to user B.
2. User A puts PeerIdOf(UserB) -> CidOf(content) to the DHT (treating the DHT as a sloppy hash table).
3. When user B comes online, they find themselves in the DHT and download their messages.
4. Finally, they send an ACK back to user A (using the same mechanism).

Unfortunately:

1. This is relying a lot on potentially ephemeral nodes.
2. There's no way to remove these messages from the DHT until they expire (see: https://github.com/libp2p/go-libp2p-kad-dht/issues/196).

No repository files, tests, or entry points are named. Start by reviewing the proposed Textile DHT flow and the linked go-libp2p-kad-dht issue, then establish an agreed design for reliable offline delivery, message removal, and acknowledgements before implementation.