Stop Listener when disposed

[brettsam]

Creating an EventHubs-specific issue for this: Azure/azure-webjobs-sdk#2823

After discussing internally with @alrod, we think that having the EH Listener also stop itself on the call to Dispose() (and skip checkpointing if disposed) is the safest path.

[alrod]

Looks like the same issue #66

[liliankasem]

@alrod Did you want to use this issue to also track changes to the service bus listener or should we create a new issue in that repo?