feat(client): add resolve timeout to HttpConnector

[chewi]

The recently-added connect timeout does not cover resolving hostnames, which
could also stall on an OS-level timeout if there are issues reaching the DNS
server.

As mentioned in #1972.

[chewi]

Ouch, serious rebase needed! I've done most of it but the pin stuff has me stumped. I'll try a bit longer.

[seanmonstar]

Sorry for missing this! Though this reminds me, once Resolve is just a Service (#1903), a user could set a timeout on it individually:

use ServiceExt;

let resolver = GaiResolver::new().timeout(some_dur);
let connector = HttpConnector::with_resolver(resolver);

[chewi]

Ah cool, I was vaguely aware that some kind of custom resolver work was going on but I didn't realise you could do that. I'll close this then but would you accept a PR to add my timeout to 0.12? I take it with 0.13 near that you won't be backporting the Service feature and we're not quite ready for 0.13 yet. -- Sent from my Android device with K-9 Mail. Please excuse my brevity.