Removed updated.filter in favor of using the updated flag in Node.
Fixed a bug where a parent of an already memo matched node could also be
matched.
Merge branch 'master' into timefree

Conflicts:
	core/src/main/scala/tbd/ddg/DDG.scala
	core/src/main/scala/tbd/ddg/Node.scala
	core/src/main/scala/tbd/worker/Worker.scala