Implement clean shutdown of Event Hubs senders

[sebastianburckhardt]

To avoid exceptions during host shutdowns, this PR adds code that cleanly terminates the event hubs senders before stopping the event hub clients.

It works as follows:

• a cancellation token is passed to the batchworker that performs the sending.
• this token is triggered as the first thing when a host is shutting down.
• from that point on, the batchworker will no longer process new events.
• at some point during shutdown, we call WaitForShutdownAsync().
• only after that do we destruct the event hub client that constructed the sender.

This guarantees that the sender is no longer active by the time we close the client.