Add IPv6 support

[Mynacol]

Currently, none of the official relay servers (smp4.simplex.im - smp10.simplex.im) have a AAAA DNS record, making them only reachable via IPv4. Adding IPv6 connectivity helps keeping long-term connections (e.g. from Android clients) alive. This allows to reduce the frequency of keepalive messages and in turn helps conserving battery space.

To deploy IPv6, the server code needs to be able to handle IP addresses in IPv6 format. Then IPv6 has to be added to the servers and announced with AAAA DNS records.
The clients may also need software changes to actually use IPv6.

[dvn0]

This is done, right?

#715

[Mynacol]

Well, support in the server software is implemented. Do you track deployment to the public relays (smp[4-10].simplex.im) elsewhere? Therefore we _could_ close this issue.

[Mynacol]

I've just noticed smp6.simplex.im and smp7.simplex.im do have AAAA DNS resource records now. It would be nice if the others get IPv6 support too, but closing as it's mostly done.