This allows Messenger to clean up processed messages from memory, avoiding a mem "leak" in redis

Right now by default a redis transport for messenger will leak memory as all messages stay in redis forever. You can configurestream_max_entries to automatically trim to a max of X entries, but that means if you have big peaks in messages you might start losing messages which have not been processed.

This PR provides an alternative to that, by deleting message as they are processed. This is ideal as it avoids having to find the right number forstream_max_entries (do you want to risk losing data or use more memory than needed on average?). The only catch is that if you have multiple groups consuming the same stream, the first one processing a message will delete it, so other groups will not see it. For that reasonsetup() attempts to detect this and fails hard if it is misconfigured to prevent data loss.