Make the ack-then-process option configurable

Historically, most of the OpenStack projects could not handle duplicated messages and would do things over
and over again (further corrupting the world). That is why olso.messaging (or impl_rabbit at least) was aiming for an
at-most-once guarantee (i.e. avoiding duplication at the expense of dropped messages.

Although, this yet to give strong guarantees for no duplicates:
- the acknowledgement is done before processing, though since the acknowledgement is asynchronous, that only
narrows the window it doesn't eliminate it.
- There is a message id 'cache' used to try and detect (and then ignore)
duplicates. It's not clear how effective that is in practice as it
only tracks the last 16 ids for a given listener. In any case if the
listener process is restarted, or if the call is redelivered to a
different server in a group, then the id cache would be of no use.

The suggested solution is to make the ack_then_process semantics to be implemented as a Work Queue API and to be adopted in the tooz & taskflow.

See http://lists.openstack.org/pipermail/openstack-dev/2015-July/068833.html
http://lists.openstack.org/pipermail/openstack-dev/2015-December/080907.html