DHT refactoring

[kurnevsky]

This PR:

• implements advanced LAN discovery from friend_connection.c
• moves onion key refreshing to dht_main_loop
• simplifies dht_server example

I decided to leave LAN discovery wake ups separated from dht_main_loop since:

• it'd have required adding additional argument to server
• LAN discovery is optional so if it's disabled we don't need LAN wake ups at all (at least in bootstrap server).

[kpp]

use select instead of join

[kurnevsky]

Could you explain why you think that select is should be used here? With select you must assume that all these futures are infinite whereas join explicitly shows that application will be terminated when all these futures are completed.

[kpp]

Because if any of the futures resolves, we should return immediately. In your case we will wait until all futures are terminated.

Right now all these futures are infinite so it is better to use .select. If the server stops with a resolved future we will notice that some of these futures resolved and stopped the server.

[kurnevsky]

But in case if one future resolves with error join won't wait other futures.
And another argument for join - select modifies error type to either of errors, so I can't just change join to select, I'll have to add map_err for every line here.

[kpp]

A simple rule:

• use select if you don't have to wait for the other futures to resolve.
• use join if you need to wait for them.

Do we need to wait for other futures?

[NamsooCho]

How about moving this macro definition to binary_io.rs.
Because this macro is used commonly in many files.