Make net::dns::resolver use concepts::io_executor

[jbaldwin]

The dns resolver hard links against coro::io_scheduler via coro::task_container<coro::io_scheduler>. Since the concepts::executor was introduced a while ago to make it possible to pass in different executors (or even user defined executors) the resolver has apparently been forgotten about. Lets introduce an io_executor (needs poll()) and make the resolver use an agnostic concepts::executor.

The other reason for doing this is that the windows build is failing when LIBCORO_FEATURE_PLATFORM=OFF and building as a SHARED library since the coro::io_scheduler is sneaking in via the resolver class but then ultimately isn't compiled since its excluded from the build.