Helen: strengthen and test message canceling behavior

[achirkin]

PROJECTS: helen
CHANGESET: Core/Service module alone = pure Haskell + monads and lenses
DESCRIPTION:

The message canceling policy in helen is not good enough (if not totally incorrect). We have two types of services: the ones that can handle new messages (non-blocking services) and the ones that cannot (blocking services).
On the other hand, a client can send message cancel: helen must guarantee that no messages from running service come after receiving cancel message.
Hence, ideal behavior is

1. If the service is non-blocking, then forward cancel message to it
2. In any case if the service sends anything related to the canceled session, send back a specific error message telling that the request was already canceled.

The corresponding piece of code is in Helen/Core/Service.hs.
The task is to modify processCancelMessage function (and some code nearby if necessary) to send make sure the behavior is correct.