[master] RFR: OMWorld: Cleanup ObjectMonitor::_header [v3]

[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.

Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:

  Use variable directly in sizeof

-------------

Changes:
  - all: https://git.openjdk.org/lilliput/pull/161/files
  - new: https://git.openjdk.org/lilliput/pull/161/files/18b0f07e..79a30ebe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=lilliput&pr=161&range=02
 - incr: https://webrevs.openjdk.org/?repo=lilliput&pr=161&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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