Add attach/detach for the dns_dispatch_send()

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.

Closes #3105 (closed)