[WIP^2] upload handler interface

refs https://github.com/owncloud/core/issues/12052#issuecomment-63972509
For MountProviders we called it "IMountProviderCollection" as it's a collection of "IMountProvider".

So here this could be called "IUploadHandlerCollection" ?
Not sure which is the most intuitive.

Anyway, I didn't dig deeper into this so you probably know better :wink: