The order in which the netievents are processed on the network manager
loop is not guaranteed.  Therefore the recv/read callback can come
earlier than the send/write callback.

The dns_request API wasn't ready for this reordering and it was
destroying the dns_request_t object before the send callback has been
called.

Add additional attach/detach in the req_send()/req_senddone() functions
to make sure we don't destroy the dns_request_t while it's still being
references by asynchronous call.