Propagate HTTP invocation context

[erwinvaneyk]

This PR addresses two things:

• The propagation of cancellation signals from the apiserver. When the HTTP connection is closed prematurely with a synchronous invocation, this is now interpreted as a signal to cancel the invocation. An explicit integration test is added to verify correct behaviour on cancellation.
• The synchronous invocation now listens for completion events of the invocation, instead of polling every 100ms. This should improve the latency of the workflow invocation as perceived by the user.

@xiekeyang - want to review this?

[xiekeyang]

@erwinvaneyk No problem, I'm taking look on it.