Batcher: Make the confirmation of successful messages more intuitive

[paul-butcher]

This business with the indices is a relic of the SQS Stream driven version. It's a bit tricky to untangle, so I've left it in place.

It might be easier to rewrite this later on when we no longer use that approach.

Originally posted by @paul-butcher in #2735 (comment)

See also Slack

How it works:

• Take a list of messages, use that to create a corresponding list of path strings
• create selectors for all the paths, associate each selector with a position in the list
• bundle up the selectors (the point of the batcher), associate each batch with a list of positions of paths in the list
• try sending on each batch (again the point of the batcher), if it fails, return the list positions that fail
• Iterate over the message list. If any message in the list has the same position as one of the paths that failed, remove that message from the output list
• Finally, emit the successful messages (or rather, the messages that did not fail), which SQSStream will then delete from the input queue.
  ...
  The way I think it should work, is that it could conceal the message within a generic, and pass that along a Flow When it succeeds, it emits whatever succeeds.

[paul-butcher]

The above statement about passing something along a Flow supposes that we continue using Akka for this, which currently looks a bit unnecessary and also contrary to the fast-start requirement of Lambda.