Call finishProcessing in the right place

We should call `InterpreterListener.finishProcessing()` only once, but
before that was not necessarily true, as it was being called inside a
loop.

Instead, now we call it in the same method as `startProcessing()`, after
all processing has been finished.

Fixes #51

src/main/java/org/graylog/plugins/pipelineprocessor/processors/PipelineInterpreter.java

@@ -265,6 +265,8 @@ public Messages process(Messages messages, InterpreterListener interpreterListen
                 }
             }
         }
+
+        interpreterListener.finishProcessing();
         // 7. return the processed messages
         return new MessageCollection(fullyProcessed);
     }
@@ -374,7 +376,6 @@ public List<Message> processForPipelines(Message message, String msgId, Set<Stri
             }
         }
 
-        interpreterListener.finishProcessing();
         // 7. return the processed messages
         return result;
     }