GH-126491: Lower heap size limit with faster marking

[markshannon]

With marking added to the cyclic GC (#127110) we spend a lot of the time in the GC forming transitive closures, both for marking and for the increments of the incremental GC.

Unfortunately the current algorithm has a couple of mistakes in it. One harmful, one beneficial.

• The beneficial one is counting the initial mark twice. This helps because it reduces the cost of GC on heaps with little or no garbage
• The harmful one is allowing the amount of work done to grow in proportion to the heap size.

These more or less cancel out.
This PR deliberately counts marking as twice as effective as normal collection, but limits the amount of work done.
To do so, we need to increase the typical amount of work done a bit.
This has the advantage of limiting the amount of garbage to (roughly) 1/3 of the heap.

This PR does two things:

• Speeds up the marking and increment creation phases
• Visits objects a bit faster to maintain a lower heap size.

• Issue: Mark all objects reachable from roots as live before doing main cyclic GC pass #126491