examples: Fix and update tcp-ack-interval example
examples: Fix and update tcp-ack-interval example

Fix botched reporting of histogram bucket counts. Logic did not take
into account that ipairs() doesn't work with non-sequential indices.
Switch to keying histogram buckets by their upper bound to avoid any
pitfalls.

Use collect() to copy histograms from worker threads to the main thread.
Display per-thread as well as one aggregated histogram.

Various renames, comments and other minor changes for readability.

jsitnicki/rushit,jsitnicki/rushit