* Initial version of gradual decommit for WKS.

This is the regions version modeled after the behavior of the segments version.

Idea is simply to limit the amount of decommitted memory based on the time since the last GC.

* Change decommit_step to take a step_milliseconds parameter - this makes the logic for the WKS decommit more straightforward.

* Only do decommits at most every 100 milliseconds to limit the number of decommitted regions.

* Address code review feedback: disable the logic in decommit_ephemeral_segment_pages for WKS, some changes in distribute_free_regions as a consequence.

* Remove unused static field ephemeral_elapsed.

* Fix typo in comment.