IPNS support

[dvc94ch]

Original from 2019-02:

- [x] resolve local records
- [x] resolve dnslink records
- [ ] resolve ipns records

Updated on 2020-09:

As the real ipns record resolving was left unimplemented and persistent "local record" support was never implemented (or maybe was, with rocksdb but it was removed in #283 for not being tested, long compilation times). In #356 we removed the in-memory only local records support but retained dnslink resolving, which is now recursive since #353.

Supporting IPNS would still be an interesting feature.

Some implementation ideas:

1. Start with reading and verifying ipns records off the DHT.
2. Re-providing the verified records (if not automatically done by libp2p-kad, which doubt is done, at least the verification part)
3. Currently not at all thought about interactions between persistence and kademlia need to be thought out
4. Once an attempt at persistence has been done the next step would easily be the implementation of locally created records and their publishing and cancellation

The first step is quite straightforward, and there is already example of using DHT to find blocks in #256, #260. The more in-depth DHT interactions become quickly difficult.

References:

dnslink: https://dnslink.io/
ipns: https://github.com/ipfs/specs/blob/master/IPNS.md, see also ipfs/specs#205
kademlia recordstore: https://docs.rs/libp2p-kad/0.23.0/libp2p_kad/record/store/trait.RecordStore.html