Support retrieving the most recent event by namespace

[psFried]

Consumers should be able to retrieve the most recent event for a given namespace (glob). This should only be allowed if there is no in-progress consumption already for that connection.

We should use an inverted index internally in the ConsumerManager to implement this. The inverted index needs to be able to grow pretty large, so this would not be trivial to write. We should look into existing libraries to use for the inverted index.