Fixes to BackgroundReaction, minor tweaks

Fixed BackgroundReaction - was previously not enforcing the rule that
the long-running background task must not perform any Box reads. This
was not a fatal flaw, but removed a check that should ensure
BackgroundReactions are better written.
BoxesDemo now correctly retains the BackgroundReaction in
backgroundReaction method.
Removed unused object DefaultMongoMetaNode.
Reduced CoalescingResponder fusion interval and tick interval, for more
responsive graphs, etc.