hammer2 - Refactor flush mechanics

hammer2 - Refactor flush mechanics

* Greatly simplify and reduce special cases in the flush code

* Remove the multi-layer rbtree and replace with two discrete rbtree's
  (rbtree and dbtree) representing the live state and one linked list (dbq)
  representing set-aside deleted chains that are not part of the live state.

* Cleanup some debugging junk, add more debugging junk.

* Separate flushing state flags and TIDs into their own fields instead of
  trying to use the live state flags and bref TIDs.

* Simplify transaction TID tracking.