RFR: 8337658: ZGC: Move soft reference handling out of the driver loop function

[Stefan Karlsson]

The ZDriver code is written to be neat and have a clear outline. The soft reference handling distracts when reading this code. I propose that we hide it a bit.

I've also clarified in comments and names that the code is dealing with clearing of *all* references.

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Commit messages:
 - 8337658: ZGC: Move soft reference handling out of the driver loop function

Changes: https://git.openjdk.org/jdk/pull/20418/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20418&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337658
  Stats: 51 lines in 8 files changed: 20 ins; 4 del; 27 mod
  Patch: https://git.openjdk.org/jdk/pull/20418.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20418/head:pull/20418

PR: https://git.openjdk.org/jdk/pull/20418