RFR: 8316299: GenShen: Reduce frequency of expedited GC

[Kelvin Nilsen]

We have found that expedited GCs too frequently yield very little benefit because they occur so soon after the previous GC that there has been no accumulation of garbage.

The primary motivation for expediting GC is to avoid a situation under which there is so much "promotion" work to be performed that the urgent need to reclaim garbage from young-generation is obstructed by this promotion work.

This PR ends the practice of expediting to support promotion in place.  The effort required to promote a region in place is minimal and unlikely to contend in a major way with young collection efforts.

This PR also reduces the likelihood that we will expedite for promotion by evacuation, because it requires the amount of accumulated promo potential to exceed a particular threshold.

-------------

Commit messages:
 - Reduce frequence of expedited GC

Changes: https://git.openjdk.org/shenandoah/pull/325/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=325&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316299
  Stats: 14 lines in 1 file changed: 2 ins; 11 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/325.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/325/head:pull/325

PR: https://git.openjdk.org/shenandoah/pull/325