Allow several callables for transfer callbacks

Idea

Currently one can only pass 1 callable (function or method) for (down|up)load callback.
An enhancement would be to allow to pass a set of callables if one wants to have more than one callback.

The retrocompatibility will be kept.

Why?

Mainly for Nuxeo Drive which have a big method as callback and where too many things are mixed. Having 2 or 3 callbacks would help cleaning up the code and make it more readable and understandable.