public void prefetch_dns (string hostname, Cancellable? cancellable, AddressCallback? callback)
Tells this that an URI from the given
hostname may be requested shortly, and so the session can
try to prepare by resolving the domain name in advance, in order to work more quickly once the URI is actually requested.
cancellable is non-%NULL, it can be used to cancel the resolution.
callback will still be invoked in this case,
with a status of CANCELLED.
a hostname to be resolved
a Cancellable object, or
callback to call with the result, or