Added a memoryEfficient mode

Added a memoryEfficient() mode, when true, it omit storing $this->results and $this->finished after processing tasks. I think this mode can be beneficial since in many case, we don't need to recheck out the result afterwards.

Use case I used your package to compute a lot of calculation with large range of numbers. So big that I had to use a php generator. After setting the pool I used htop to look at memory usage and it was going up and up, until hitting memory limit (more than 5Go !) After my PR, the memory usage stay stable.

Example usage

function rangeGenerator($start, $end)
{
    for ($i = $start; $i <= $end; $i++) {
        yield $I;
    }
}

$pool = Pool::create()
    ->memoryEfficient();
    ->concurrency(32);

$find_min = null;
foreach (rangeGenerator(0, 10000000) as $seed) {
  $pool
    ->add(function () use ($seed) {
      // Calculation related to $seed

      return $result;
    })
    ->then(function ($output) use (&$find_min) {
      if (is_null($find_min) || $output < $find_min) {
        $find_min = $output;
        echo "New min : " . $find_min . "\n";
      }
    });
}
echo "Result : " . $find_min . "\n";

Side note: Should we throw an exception on getFinished() if memoryEfficient is set to true ?

spatie / async

Added a memoryEfficient mode #219