RFR: 8256401: ZGC: Improve ZList verification

[Per Liden]

The ZList verification is currently fairly weak. I propose that we make the verification stronger to easier detect corrupt lists and misuse of list nodes. This patch also removes the unused `ZList::transfer()` function.

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Commit messages:
 - 8256401: ZGC: Improve ZList verification

Changes: https://git.openjdk.java.net/jdk/pull/1230/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1230&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256401
  Stats: 170 lines in 4 files changed: 32 ins; 105 del; 33 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1230.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1230/head:pull/1230

PR: https://git.openjdk.java.net/jdk/pull/1230