RFR: 8269914: Factor out heap printing for G1 young and full gc

[Thomas Schatzl]

Hi all,

can I have reviews for this change that factors out before/after heap related printing into a single scoped object to be used for both young and full gc code?

There is some minor change in order of printing of unrelated log output (COMMIT/UNCOMMIT region state changes vs. other heap printing), during full gc but since this is very low level debugging I do not see a problem to do that.

(Sorry for the trouble with open/closing this issue already: have been experimenting with the feature to base PRs on other PRs which kind-of failed a bit; rebased on tip after being able to push all dependencies, so this one should be good)

Thanks,
  Thomas

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Commit messages:
 - Add missing code
 - Initial implementation of heap printing refactoring
 - Fix G1AllowPreventiveGC -> G1UsePreventiveGC
 - lkorinth review
 - Initial implementation

Changes: https://git.openjdk.java.net/jdk/pull/4705/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4705&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269914
  Stats: 166 lines in 6 files changed: 121 ins; 39 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4705.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4705/head:pull/4705

PR: https://git.openjdk.java.net/jdk/pull/4705