Add REST API cache busting mechanism.

[zinigor]

Fixes #5217

Changes proposed in this Pull Request:

• Refactored the REST API component to make it more DRY.
• Adds a cache busting mechanism to address the problem of requests being cached.

Testing instructions:

• I had a really hard time reproducing the problem on my own server, but it could be reproduced on SiteGround shared hosting. If you enable the browser console and allow browser caching in the settings, you will see that some requests come back with 304 Not Modified. My guess is that it's because the server sends back the Last-Modified header with the zero epoch time every time. If you can't reproduce it, try adding the header to API responses manually with this:

add_filter( 'rest_post_dispatch', function( $response, $server, $request ) {
	$response->set_headers(
		array_merge(
			$response->get_headers(),
			array(
				'Last-Modified' => date( 'r', 0 )
			)
		)
	);
	return $response;
}, 3, 10 );

• See that the requests are cached.
• Update to this PR.
• See that the cache busting mechanism defeats the problem.

Proposed changelog entry for your changes:

• Fixed the problem of API requests being cached on some hosts.

[enejb]

I don't think it is necessary to cache bust POST requests. Browsers don't cache them by default.

[enejb]

It would be good to pass in the getParams explicitly into the function. So that the code is more readable.

[enejb]

instead of doing generating a random math character. I would suggest we just use the current time in seconds or milliseconds with

var ddate = new Date();
var milliseconds = d.getTime();

also can we make the _rnd more explicit with something like _cacheBuster

[dereksmart]

I like enej's suggestions, but otherwise this is a nice change 👍

[zinigor]

Thanks for the review @enejb, @dereksmart ! I have made changes to account for all the comments.

[dereksmart]

Changes look good! Let's get this in

[eliorivero]

Looks good!