Fix ownership on 'NextCallback'

[arteymix]

One big drawback of not passing the 'NextCallback' ownership forward is that it cannot be called after the callee has returned (e.g. in a thread or asynchronously).

In the short-term, this is going to be really convenient to use threads for processing requests. It is necessary to build a threading middleware that would dispatch requests handling in a pool.

It's still bugging with the context being null, so there's some investigation to be performed.