Processing Pipeline loop make Graylog unusable

## Current Behavior
Create a rule that creates a new message from each incoming message and route that to the same stream the rule is connected to.

```
rule "clone and route"
when
true
then
let mess = clone_message();
route_to_stream(name: \"anonyme\", message: mess);
end
```

## Context
This is more or less wanted behaviour, but maybe we identify/create a solution that routing to the same stream a rule/pipeline is connected to does throw an error. Just because it is to easy to make Graylog unusable.

After some time the Webinterface will not response and you need to remove the rule in the Database direct to let Graylog return to a functional state.

## Your Environment

* Graylog Version: 2.4.3