[GC] Backport JEP346 Promptly Return Unused Committed Memory from G1

[leveretconey]

Summary: Backport JEP346 Promptly Return Unused Committed Memory from G1

Test Plan: jtreg/gc/g1

Reviewed-by: mmyxym, linade

Issue: https://github.com/dragonwell-project/dragonwell11/issues/662

[CLAassistant]

All committers have signed the CLA.

[linade]

Do we expect a lot of trouble when merging this back to ajdk?

[leveretconey]

@linade I think there will be some trouble when we merge this back. I'm not sure if we have any method to avoid that trouble.