Add ext-async into benchmark

[dbalabka]

Prepare additional benchmark with:

1. https://github.com/concurrent-php/ext-async
2. https://github.com/amphp/amp/tree/ext-async
3. https://github.com/concurrent-php/http

Extension instalation example:
https://github.com/concurrent-php/async-api/blob/master/vagrant-async.sh#L46