Switch I/O polling from `pending_req_poll_v2` to `select`

Related to #213, we should switch from our current use of the `fastly_http_req::pending_req_poll_v2` hostcall + `time.Sleep()` to the newer `fastly_async_io::select` hostcall which takes a timeout.  That would mean spending more waiting in hostcalls if there's no I/O but I think the tradeoff of more responsive I/O is a bigger win than goroutine responsiveness.