RFR: 8366363: MemBaseline accesses VMT without using lock [v12]

[Johan Sjölen]

> Hi,
> 
> The `MemBaseline` used to access the VMT instance directly without a lock. We fix that, and we switch from using a `LinkedList` to a copied `RegionsTree` instead.

Johan Sjölen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 16 commits:

 - Merge remote-tracking branch 'openjdk/master' into cleanup-membaseline
 - Rename to _vma_allocations
 - After a lot of casting
 - Yup
 - Some more ugly casts
 - Aha, _root is typed as an IntrusiveRBNode
 - Aha, missing include
 - Wait, the VMATree is wrong?
 - Rename
 - It's const
 - ... and 6 more: https://git.openjdk.org/jdk/compare/6dda2f6f...82df750b

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

Changes: https://git.openjdk.org/jdk/pull/27003/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27003&range=11
  Stats: 226 lines in 11 files changed: 161 ins; 44 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/27003.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27003/head:pull/27003

PR: https://git.openjdk.org/jdk/pull/27003