RFR: 8366363: MemBaseline accesses VMT without using lock [v4]

[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 refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR. The pull request contains one new commit since the last revision:

  Rewrite copying entirely

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27003/files
  - new: https://git.openjdk.org/jdk/pull/27003/files/800eca26..2e7bd8c9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27003&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27003&range=02-03

  Stats: 80 lines in 5 files changed: 40 ins; 18 del; 22 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