Support Promise cancellation

[clue]

We should register a Promise cancellation handler so that the following code actually cleans up the underlying socket resource:

$promise = $resolver->resolve('reactphp.org');

$promise->cancel();

(This likely depends on #19)

[clue]

This also likely depends on reactphp-legacy/socket-client#40 so that a (pending) TCP connection to the DNS server can also be canceled.