This function has a bug when `last` returns npos (which then fails an
assertion in a debug build).
Rather than fixing it, it seemed easier just to remove it and rely on
the split() version that does almost exactly the same thing but takes a
string_view delimiter.
* make it so that we don't set up unbound resolver when we have no resolvers provided by config
* clean up dns codepath and make it use llarp::SockAddr instead of llarp::IpAddress