Asynchronously retrieves the next SocketAddress from
this and then calls callback, which must call next_async.end
to get the result.
It is an error to call this multiple times before the previous callback has finished.
| this | |
| cancellable |
optional Cancellable object, null to ignore. |
| callback |
a TaskReadyCallback to call when the request is satisfied |
| user_data |
the data to pass to callback function |