[gclist] Baker's treadmill improvments.

[Nick Barnes]

> First applications that use the treadmill will be giving the collector 
> atoms of time when they are idle. This more than fills the collectors 
> needs for time and can be used to finalize and free objects instead of 
> having that work done when a header is needed.

Many applications are never idle, or need to be able to run for hours
or days at a time with no idle time. Should these applications not use
the treadmill?

Nick B