Use reactphp/async instead of clue/reactphp-block

[dinooo13]

See https://github.com/reactphp/async

Ref: clue/reactphp-block#67, clue/reactphp-block#68, reactphp/http#464 and clue/reactphp-mq#34

[SimonFrings]

Just a minor thing I noticed

[SimonFrings]

Pull request here looks good, let's get this in 👍

[SimonFrings]

@dinooo13 Just noticed, the test suite is currently not running through as it can be seen in #53 and we need to update our test suite. @yadaiio created a pull request for this in #54. Once this is merged we have to come back to this one here and rebase once. Additionally, we'll probably encounter a conflict in the composer.json file when rebasing, as #54 updates the PHPUnit versions which are right below you changes.

I will give an update in here once #54 is merged so we can move on and get this pull request in 👍

[SimonFrings]

@dinooo13 With #54 merged I think we can rebase in here and then get this in 👍