[11u] Backport JEP 346 : Promptly Return Unused Committed Memory from G1

[Ruslan Synytsky]

Hi everyone, I believe backporting JEP 346 to JDK 11 is a great idea. The
first version of JEP 346 was against JDK 11, but then due to timing it got
pushed to JDK 12. This improvement is very useful for saving resources and
for making java more elastic - no need to use dirty hacks with an agent and
explicit call of Full GC.

We had a conversation in another thread about this backport and Thomas
Schatzl kindly agreed to help with review, but looks like we need to get
pre-approval from the current maintainers, so we do not waste our time for
nothing at the end.

Thank you and stay safe
-- 
Ruslan Synytsky
CEO @ Jelastic Multi-Cloud PaaS