RFR: 8257774: G1: Trigger collect when free region count drops below threshold to prevent evacuation failures

[Aditya Mandaleeka]

_This PR picks up from [this previous PR](https://github.com/openjdk/jdk/pull/1650) by @charliegracie._

I won't repeat the full description from the prior PR, but the general idea is to add the notion of a "proactive GC" to G1 which gets triggered in the slow allocation path if the number of free regions drops below the amount that would be required to complete a GC if it happened at that moment. The threshold is based on the survival rates from eden and survivor spaces along with the space required for tenured space evacuations.

There are a couple of outstanding issues/questions known:
- _Interaction with GCLocker_: In the case where we determine that a proactive GC is required and GC locker is active, we don't allow the young gen to expand (instead threads will stall). @tschatzl raised this and suggested that it should be discussed as part of the review.
- _Disable proactive GC heuristic during initialization_: I ran into an issue in testing where code in the universe initialization codepath was tripping the proactive GC heuristic, leading to a GC being triggered before the VM has finished initialization. I need to find a good way to prevent this from happening. There may already be a mechanism for this, but I couldn't find one so I added a temporary placeholder (`zzinit_complete`) just to unblock testing.

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Commit messages:
 - Fix whitespace and comment.
 - Further refactoring/changes.
 - Address feedback and make other changes.
 - Improve G1GC accounting for humongous objects

Changes: https://git.openjdk.java.net/jdk/pull/3143/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3143&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257774
  Stats: 218 lines in 13 files changed: 163 ins; 4 del; 51 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3143.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3143/head:pull/3143

PR: https://git.openjdk.java.net/jdk/pull/3143