This is a similar idea to a sync.WorkGroup except that it tracks the
completion of individual items represented by comparable values, rather
than only a count of expected items to complete.
My intention for this is for the planning engine to use it to track when
all of the expected work for a provider instance or ephemeral resource
instance has been completed, so that it can be closed shortly afterwards
instead of waiting for the entire planning operation to run to completion.
Signed-off-by: Martin Atkins <mart@degeneration.co.uk>
