Fix a non-deterministic integration test

There was no guarantee that the messages would be evenly divided between the three buckets, but that was what the test was asserting. This version just checks that at least one bucket got at least one message – because the point of the test is just to show that custom demultiplexers can be used, not that the specific demultiplexer works.
Remove an empty spec file

It’s been here since the first commit, empty.
Apply some small optimizations to QueueingConsumer#deliver

Avoid calling methods multiple times, basically.
Add a test for the handling of empty batches

And remove the comment.