Pool.php - add methods to clear finished/results cache (spatie#235)

Since the member variables of finished/results keep growing indefinitely over time this is a source of a memory leak.
This memory leak is resolvable by clearing the result caches with the newly introduced methods.

src/Pool.php

@@ -367,4 +367,15 @@ public function stop()
     {
         $this->stopped = true;
     }
+
+    public function clearFinished()
+    {
+        $this->finished = [];
+    }
+
+    public function clearResults()
+    {
+        $this->results = [];
+    }
+
 }