The [original commit to remove the `sleep` call][1], only considered newly-arriving
messages; but didn't take into account that existing messages could become visible
in a much shorter time duration than the wait timeout. This commit addresses this
by checking for newly-visible messages at the visibility timeout granularity (seconds),
which is still a major improvement over the original 10 millisecond polling.
[1]: <d560ff002d> "Remove Sleep Call"
7c44e48670