Fix for case where a TopicActor can continue sending messages to a member that no longer exists

[benbenwilde]

Description

We had an issue where a client member would be shutdown, but a remaining member would keep trying to connect to it and failing forever. This was due to a subscription in the TopicActor to that removed member. Since it was a client member, it is not detected as a removal from the topology, which the TopicActor already watches to unsubscribe members that have shutdown but failed to unsubscribe.

This change adds a handler for DeadLetterResponses to the TopicActor in order to detect when it is no longer able to reach a node with subscriptions, which then removes the subscriptions for that node. A test that validates the new code is included.

A minor change has been added to the BatchingProducer as well so it can stop more quickly and cleanly when it's being disposed.

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)