[WIP] introduction of interface IChunkHandler and a very first stupid implemen...

...tation reusing the existing chunking mechanism

:construction: 

refs https://github.com/owncloud/core/issues/4997

@icewind1991 @PVince81 
I see, so the last chunk would trigger assembling.

I'm not sure, I'd have moved this to a separate function but am not sure why. Maybe to give the caller more control of when they want to do the assembling.

If you decide to keep it this way, it might be a good idea to at least expose `isComplete()` on the interface.