* Added shutdown() functions to SmtEngine, TheoryEngine, PropEngine,

* Added shutdown() functions to SmtEngine, TheoryEngine, PropEngine,
  DecisionEngine, and Theory.  These are triggered from the SmtEngine
  dtor before the other engines are deleted.  This is important
  because of potential issues with outstanding TNodes in Theory
  implementations (which fail if the destruction is done in the wrong
  order, in certain cases).

* Favor "FactQueueResetter" instead of clearAssertionQueues() for
  resetting facts queue in Theory implementations.

* Better theory-rewriting code.

* Minor cleanups.