Improve the performance of ObjectFieldsAppendingMarker 

Improve the performance of `ObjectFieldsAppendingMarker` (30% faster) by avoiding the conversion
of the object to a `JsonNode` tree prior to serializing.

Instead, now an `UnwrappingBeanSerializer` is used to write the object directly
to the `JsonGenerator` (without having to convert to `JsonNode`s first).

Now, the `ObjectFieldsAppendingMarker` is actually faster than the `MapEntriesAppendingMarker`
when you want to embed fields from an object in the logstash event json.