Investigate curl_multi for async

[dcramer]

It seems like we could use the curl_multi stuff to do async http requests with fairly large compatibility

http://us3.php.net/manual/en/function.curl-multi-init.php

[henrikbjorn]

https://github.com/airbrake/airbrake-php/blob/master/src/Airbrake/Connection.php#L83

[filipegiusti]

@henrikbjorn seems not a good approach, check https://segment.io/blog/how-to-make-async-requests-in-php/.

[dcramer]

We can likely build off of Phabricator's HTTPSFuture for this:

https://secure.phabricator.com/diffusion/PHU/browse/master/src/future/http/HTTPSFuture.php$10

[ste93cry]

Closing this as it's been fixed in #132