New ContinuationCallback interface

[Sergey Beryozkin <sb...@gmail.com>]

Hi

I'd like to add a new ContinuationCallback [1] interface for getting it 
easier to track various invocation status updates.

Particularly it would make it possible for the JAX-RS frontend to 
support CompletionCallback [2] but I think it can also be of use to CXF 
applications not relying on JAX-RS.

At the moment this interface is a work in progress, supports only two 
callback methods. We can improve that when we have more ideas what to 
make available to the specific callback methods...

Ideally I'd like to merge it now, before 2.7.0 is out, this is the last 
bit I'd like to do for JAX-RS 2.0 AsyncResponse and then move to working 
on other things... The code seems safe as the continuation based tests 
are running OK, but I can wait till after 2.7.0 is out too

Cheers, Sergey



[1] https://issues.apache.org/jira/browse/CXF-4531
[2] 
http://jax-rs-spec.java.net/nonav/2.0-SNAPSHOT/apidocs/javax/ws/rs/container/CompletionCallback.html