RFR: 8316299: GenShen: Reduce frequency of expedited GC [v2]

[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.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Remove ShenandoahHeap::[gs]et_promotion_in_place_potential() everywhere

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/325/files
  - new: https://git.openjdk.org/shenandoah/pull/325/files/44b203a0..de00edc3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=325&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=325&range=00-01

  Stats: 17 lines in 5 files changed: 0 ins; 17 del; 0 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