Add Support for curl_multi_add_handle

[ruflin]

Elastica should support curl multi handle as does the elasticsearch-php client. More details see here: https://github.com/elastic/elasticsearch-php/blob/de7c2694a9d63a8f21cdda5a286aacc901fb42f9/src/Elasticsearch/Connections/CurlMultiConnection.php#L168

[im-denisenko]

@ruflin I read carefully this code, but don't really get that exactly they achieve using curl in multi mode, but sending just one request at time. Could you explain please why we also should switch to this way? :)

[ruflin]

I would not "switch" but also support it. The way I understand multi curl is that it allows to add multiple requests which are then handled asynchronously. This means multiple single "requests" could be sent even to different hosts and waiting for the responses would be done by curl_multi. This would bring an improvement in speed as multiple queries at once could be fired off. What kind of magic curl_multi does in the background I also don't know. Probably we should find out :-)

[ruflin]

This would get obsolete with #944

[sidz]

We no longer have own transports.