Added debugging tools. Bugfix.

- We now write a paper-trail of where datasets are read or written to the "flow" log.
- We replaced the legacy logic in get_invocation_flat_list script with the functionality from the *trace* module.
- We added logging at the KV-queue level (but we added the stuff above and found them more useful).
- We implemented separate node and directory "wait" calls on the KV, and the KV-queue.
- We were incorrectly storing a particular class of reduction results as a "POST-COMBINE" dataset instead of "POST-REDUCE".

* The "KV queue" abstraction is different that the distributed, operational queue.