RFR: JDK-8269423: ZGC: Support pinned Object and remove/simplify GCLocker usage in ZGC

[Hamlin Li]

This PR is not an real intention to push current implementation, and current implementation does not support ZVerifyViews (which was pointed out by Per at https://github.com/openjdk/jdk/pull/4638#issuecomment-872033165).

Let's see if this solution is feasible and how to support ZVerifyViews efficiently.

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Commit messages:
 - ZGC supports obj pinning

Changes: https://git.openjdk.java.net/jdk/pull/4650/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4650&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269423
  Stats: 175 lines in 13 files changed: 100 ins; 64 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4650.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4650/head:pull/4650

PR: https://git.openjdk.java.net/jdk/pull/4650