* previous implementation was a bit hastily implemented and
  much more complicated than needed:
  trying to support concurrency but not doing it well, while
  we don't actually need any of it.
* unit testing was very flakey, in particular in circleCI there's
  lots of timing inaccuracy leading to broken tests
* it was inaccurate, for shortlived operations it would be negligible
  but for longer operations it is not. While our use case operations
  should be short, it is nice to be able to fix this while we're at it.

So now we simplify the design a lot, basically reduce it to
a non-concurrent accounting structure, removing all channels,
goroutines and context, making the accounting more correct
and the testing more robust.