RFR: 8254672: ZGC: ZParallelOopsDo/ZSerialWeakOopsDo should use atomic load/store

[Per Liden]

ZParallelOopsDo/ZSerialWeakOopsDo should use atomic load/store. This patch builds on the JDK-8254671, which is
currently out for review, so only the last commit is part of this PR.

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Commit messages:
 - 8254672: ZGC: ZParallelOopsDo/ZSerialWeakOopsDo should use atomic load/store
 - 8254671: ZGC: Remove unused roots iterator types

Changes: https://git.openjdk.java.net/jdk/pull/630/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=630&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254672
  Stats: 54 lines in 2 files changed: 0 ins; 50 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/630.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/630/head:pull/630

PR: https://git.openjdk.java.net/jdk/pull/630