As long as we guarantee that the execution order for the post ops are
upheld, we can allow insertion of multiple ops on the same key.
Implemented by adding a ref count to each position.
The restriction then becomes that an added key must either be larger
than any already existing key, _OR_ already exist. In the latter case,
we still know that we have not finished this position and signaled
"upwards".
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.
