Previous version dit looping on post execution and signaling of waiters.
This could "race" with an op just finishing if task reordering happened.
This version simplifies the code significantly (and raises the question why
it was not written like this in the first place... Shame on me) by simpy
building a promise-dependency chain between _previous_ queue items and next
instead.
Also, the code now handles propagation of return value from the "Func" pre-op
to the "Post" op, with exceptions automatically handled.
Small utility to order operation->post operation
so that the "post" step is guaranteed to only be run
when all "post"-ops for lower valued keys (T) have been completed
This is a generalized utility mainly to be testable.
