Release memory to OS on garbage collection

ZMM often holds onto big chunks of memory after they aren't used anymore, which is fine in a webserver, but it's not OK for PM.

src/pocketmine/scheduler/GarbageCollectionTask.php

@@ -25,11 +25,13 @@ namespace pocketmine\scheduler;
 
 use function gc_collect_cycles;
 use function gc_enable;
+use function gc_mem_caches;
 
 class GarbageCollectionTask extends AsyncTask{
 
 	public function onRun(){
 		gc_enable();
 		gc_collect_cycles();
+		gc_mem_caches();
 	}
 }