Cyclic reference with loop, resolver will never be destructed about dns

[kelunik]

I've just released v0.9.10 with the fix.

[kelunik]

This is a valid concern, as a DNS resolver might be used standalone without relying on the global instance.

[brstgt]

Also it is not possible to correctly clean loop/resolver instances e.g. when it is replaced with another instance which happens massively in tests.

[brstgt]

Tested and works for manual cleanup:

    public function shutdown() {
        if ($this->gcWatcher !== null) {
            Loop::cancel($this->gcWatcher);
            $this->gcWatcher = null;
        }
    }

    public function __destruct() {
        $this->shutdown();
    }

[kelunik]

@brstgt Cyclic references shouldn't matter in tests and should be cleaned up correctly during the loop switch.