[lworld] RFR: 8271910: [lworld] Heap functions/callbacks should support value fields

[Alex Menkov]

The fix implements support for flat value object by heapwalking API.

FollowReferences/IterateThroughHeap are executed at safepoint and agent callbacks may request to set/update tags for the reported object. To store tags for flattened object at safepoint a special tag table is used (entries contain strong references to holder objects), entries from the table are converted to entries in main tag table outside of safepoint.

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

Commit messages:
 - jcheck
 - heapwalking

Changes: https://git.openjdk.org/valhalla/pull/1576/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1576&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8271910
  Stats: 1749 lines in 6 files changed: 1405 ins; 102 del; 242 mod
  Patch: https://git.openjdk.org/valhalla/pull/1576.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1576/head:pull/1576

PR: https://git.openjdk.org/valhalla/pull/1576