[master] RFR: OMWorld: Cleanup ObjectMonitor::_header

[Axel Boldt-Christmas]

This changes the `ObjectMonitor::_header` to `ObjectMonitor::_metadata` and cleans up the surrounding terminology. 

Parts of this could be upstreamed, but the distinction of this field between the different locking modes is introduced in OMWorld. 

This patch will conflict with Lilliput if rebased on-top of OMWorld because Lilliput introduces `static ByteSize header_offset()` which is not used after OMWorld.  They could probably be removed in both these patches.

This also changes all the fields to have the same access level (private) so that this can be a standard layout type.

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Commit messages:
 - Cleanup ObjectMonitor::_header

Changes: https://git.openjdk.org/lilliput/pull/161/files
  Webrev: https://webrevs.openjdk.org/?repo=lilliput&pr=161&range=00
  Stats: 95 lines in 8 files changed: 23 ins; 10 del; 62 mod
  Patch: https://git.openjdk.org/lilliput/pull/161.diff
  Fetch: git fetch https://git.openjdk.org/lilliput.git pull/161/head:pull/161

PR: https://git.openjdk.org/lilliput/pull/161