This branch updates the linkerd2-proxy-dns crate to std::future and
the latest release of trust-dns-resolver, and updates the uses of DNS
in linkerd2-proxy-app to track the changes.

For the most part, this change is pretty straightforward. However, the
std::future version of trust-dns-resolver changed how the background
task driving DNS resolution is executed. Previously, it was returned by
the AsyncResolver constructor, but now, the constructors are async fns that implicitly spawn the background task on the runtime that
executes them.

Therefore, I had to change how the DNS resolver is constructed. I opted
to build the admin runtime early and have it block on the DNS resolver
construction, and then pass that runtime to the admin thread when it is
spawned. This seemed better than making the DNS resolver lazy by making
the underlying trust-dns async resolver an Option or something, and
having it panic when the resolver hasn't been constructed yet. Doing
that safely would have required adding a RwLock or something, which
didn't seem ideal.