Update Concurrency course with times (#2007)

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 :)

src/concurrency/async-control-flow.md

@@ -0,0 +1,3 @@
+# Channels and Control Flow
+
+{{%segment outline}}