Perform listener DNS lookups asynchronously, with a timeout

[teor2345]

Is your feature request related to a problem? Please describe.

Zebra uses serde to convert its listener addresses into SocketAddrs. Serde uses Rust's standard to_socket_addrs function to convert strings to IP addresses. If the strings are hostnames, to_socket_addrs uses DNS for the lookup.

If a user configures a DNS name as one of their listener addresses, and DNS takes a long time to resolve, Zebra can hang when loading the config. Since these DNS calls are blocking, and tokio uses cooperative multitasking for async code, this can cause arbitrary parts of Zebra to hang.

Describe the solution you'd like

Zebra should declare all its configured addresses as strings, then convert them using tokio::net::lookup_host().await?:
https://docs.rs/tokio/1.1.0/tokio/net/fn.lookup_host.html

lookup_host correctly wraps the DNS call in spawn_blocking, which avoids blocking other tasks.

We should also apply a timeout of at least 5 seconds to each DNS lookup:
https://tools.ietf.org/rfcmarkup?doc=1123#section-6.1.3.3

Describe alternatives you've considered

We could do nothing, leading to a poor user experience for users that use dynamic DNS and similar protocols.

We could change the function that serde uses to deserialise, but I'm not sure how serde interacts with async code.

Additional context

We want to do a similar fix for the DNS seeder addresses in PR #1662.
Also see this comment: https://github.com/ZcashFoundation/zebra/pull/1535/files#r563402763

This task is a lower priority, because most users use IP addresses for listener ports.

[teor2345]

This ticket is about listener ports, most people don't use DNS to configure them.

#1613 is about initial peers, which use DNS seeders by default.

[teor2345]

This doesn't seem to be an issue in practice, we can fix it if it becomes one.