Implement support for parallel requests

[user52]

Implement support for parallel requests. Use curl_multi_init and see examples of usage

https://github.com/petewarden/ParallelCurl/blob/master/parallelcurl.php
https://github.com/dulao5/parallel-curl
https://github.com/stil/curl-easy
https://github.com/jmathai/php-multi-curl/blob/master/EpiCurl.php
http://php.net/manual/en/function.curl-multi-init.php

[zachborboa]

Possible implementation.

Call one url with the multiple data sets (one to many):

$curl = new Curl();
$curl->get(array(
    'https://www.example.com/search',
), array(
    'q' => 'first search',
), array(
    'q' => 'search #2',
), array(
    'q' => '3rd search',
));

Call multiple urls all with the same data (many to one):

$curl = new Curl();
$curl->get(array(
    'https://duckduckgo.com/',
    'https://search.yahoo.com/search',
    'https://www.bing.com/search',
    'http://www.dogpile.com/search/web',
    'https://www.google.com/search',
    'https://www.wolframalpha.com/input/',
), array(
    'q' => 'first search',
));

Call multiple urls with the multiple data sets (many to many).

$curl = new Curl();
$curl->get(array(
    'https://duckduckgo.com/',
    'https://search.yahoo.com/search',
    'https://www.bing.com/search',
    'http://www.dogpile.com/search/web',
    'https://www.google.com/search',
    'https://www.wolframalpha.com/input/',
), array(
    'q' => 'first search',
), array(
    'q' => 'search #2',
), array(
    'q' => '3rd search',
));

Call a url with data (one to one):

$curl = new Curl();
$curl->get('https://www.example.com/search', array(
    'q' => 'keyword',
));

[zachborboa]

Implementation with success, error, and complete callbacks.

$curl = new Curl();
$curl->success(function($instance) {
    echo 'call was successful.' . "\n";
    echo 'response was' . "\n";
    echo $instance->response . "\n";
});
$curl->error(function($instance) {
    echo 'call was unsuccessful.' . "\n";
    echo 'error code:' . $instance->error_code . "\n";
    echo 'error message:' . $instance->error_message . "\n";
});
$curl->complete(function($instance) {
    echo 'call completed' . "\n";
});
$curl->get(array(
    'https://www.example.com/search',
), array(
    'q' => 'first search',
), array(
    'q' => 'search #2',
), array(
    'q' => '3rd search',
));

[zachborboa]

Calling get() in parallel is now supported. TODO: Implement post(), put(), patch(), and delete() in parallel #14.