Fixes#2060. Note that this does not change the "blocking executor"
example because on that slide it is worthwhile to sleep for 1 * 10ms on
the first iteration and so on. But we shouldn't use one-indexed
inclusive loops when the only significant feature of the loop is its
iteration count.
As I mentioned in #1536:
* Break into segments at approximately the places @fw-immunant put
breaks
* Move all of the files into `src/concurrency`
* Add timings and segment/session metadata so course outlines appear
There's room for more work here, including some additional feedback from
@fw-immunant after the session I observed, but let's do one step at a
time :)
