Moved LongPollDelay setting to IConfigurationManager.
- Moved the longpolling specific setting to be with the other
  configuration objects.
Handle canceled tasks in the Subscription and MessageBroker.
- Treat canceled tasks like faulted tasks.

#2302
Added another test to verify cancellation handling.
Removed useless argument from ConnectionExtensions.Send.

#2330
Merge branch 'release' into dev