Warp: enriching server-triggered (push/websockets) events

When testing server-side Push mechanism (like Atmosphere), the request starts on the server (not on the client as in client-server architectures).

This is fundamentally changing how the synchronous Warp tests works:

We need to install the assertion pernamently on server side and filter actions we are interested in.

Then, once the event happens, we need to transport it locally (see back-channel proposal ARQ-1005), where we can register result of assertion and/or trigger client-side callback.