Improve memory consumption by cleaning up unneeded references

[clue]

While debugging some very odd memory issues in a live application, I noticed that this component shows some unexpected memory consumption and memory would not immediately be freed as expected. Let's not call this a "memory leak", because memory was eventually freed, but this clearly caused some unexpected and significant memory growth.

One of the core issues has been located and addressed via reactphp/event-loop#164, but even with that patch applied the resolve() method behaved a bit unexpected.

I've used the following script to demonstrate unreasonable memory growth:

<?php

use React\EventLoop\Factory;

require __DIR__ . '/../vendor/autoload.php';

$loop = Factory::create();

$loop->addPeriodicTimer(0.001, function () use ($loop) {
    $promise = \React\Promise\Timer\resolve(60.0, $loop);
    $promise->cancel();
});

$loop->addPeriodicTimer(1.0, function () {
    echo memory_get_usage() . PHP_EOL;
});

$loop->run();

Initially this peaked at around 320 MB on my system. After applying the referenced patch, this went down significantly and fluctuated somewhere between 1 MB and 12 MB. After applying this patch, this script reports a constant memory consumption of around 0.7 MB.

This implementation includes some of the ideas discussed in reactphp/promise#46 and reactphp/socket#113. Eventually, we should look into providing a way to address this within our promise implementation.

My vote would to be get this in here now as it addresses a relevant memory issue and eventually address this in the upstream component (at which point this changeset also does no harm).