Allow Handlers to step back from processing

[haasalex]

Solution for #446

[pestaa]

What's holding this back?

[goetas]

Sorry for the terribly long feedback loop

If you have subscribed to handle the serialization, you should know what to do.
You can always redirect the serialization to a new type, see https://github.com/schmittjoh/serializer/blob/master/src/JMS/Serializer/Handler/PropelCollectionHandler.php#L65 for an example

[goetas]

duplicate of #177 (has tests)