v3: consider merging DHT and Relay clients

[Nuhvi]

Clients can use relays and DHT in parallel, which at least has the advantages:

1. working even in networks that blocks UDP and or Bittorrent-like packets
2. using the DHT even if you trust a relay increases the censorship resistance
3. might make resolvers redundant? but only if the user doesn't disable relay feature
4. Massively simplifies the extra features since there will only be one Client struct instead of two in native.

Cons:

1. Redundant bandwith consumption, especially when you use the same server as a resolver and a relay.
2. Harder to debug.
3. Harder to use a testnet, since you have to override default relays too.
4. Dependency on Tokio runtime unless you disable the relay feature.

Using relays can be done in parallel, or only when UDP issues are detected, although that is easier said than done.