Support realistic display delays (for debugging).

ASDISPLAYNODE_DELAY_DISPLAY unconditionally delays *all* attempts to
execute display blocks, even those that have been cancelled.  Fix that.

The DELAY_DISPLAY debugging tool is intended to simulate the effect of
expensive draw calls -- it lets you test your app's behaviour under
conditions potentially even worse than an iPhone 4.  Unfortunately, the
current simulation is unusably incorrect, as can be demonstrated by
repeatedly scrolling up and down in the Kittens example project.

ASRangeController enqueues asynchronous display of ASTableView cell
nodes when they enter the working range, and suspends their display when
they exit.  On a slow device, this performs as desired, limiting
expensive display computation to the space of the working range.  In
the DELAY_DISPLAY simulation, the suspended displays are as expensive as
the executed ones, so ASRangeController becomes unusably slow.