Adding un-ack functionality to ObservableQueue

[YoBeMo]

The default event process doesn't act on unhandled exceptions.

That means that in some use cases messages are de-queued, consumed by Conductor and fail somewhere in the process.
In such cases, no action is made against the queue message itself, meaning that a message can remain un-acked.

In AMQP, messages that are extracted from a queue and are not ack\nacked will result in channel disconnection after some interval (the default is 30 min).
We thought about adding a nack functionality, so in such cases, a message will be nacked and would not remain hanging without any proper handling.

What do you think about this approach?

[v1r3n]

Hi @YoBeMo this is a good approach, feel free to a send a PR.
Relevant issue:
#44