Add example with many process pools, to test proper garbage collection

examples/5-many-pools.php

@@ -0,0 +1,21 @@
+<?php
+
+require __DIR__ . '/../vendor/autoload.php';
+
+use Amp\Parallel\Worker\DefaultPool;
+use function Amp\ParallelFunctions\parallelMap;
+use function Amp\Promise\wait;
+
+// This pool uses 30 separate process pools in a row and processes 50 tasks for each.
+// It demonstrates that new pools can be used without issues. For better efficiency,
+// always use the same process pool, which can simply be the default pool.
+
+for ($i = 0; $i < 30; $i++) {
+    $pool = new DefaultPool();
+
+    $promises = parallelMap(range(1, 50), function () {
+        return 2;
+    }, $pool);
+
+    $result = wait($promises);
+}