[foreign-jextract] RFR: 8253823: Investigate ways to make handoff-like operation more explicit (foreign-jextract edition)

[Maurizio Cimadamore]

This patch replaces all references to `withOwnerThread(null)` with the new `share()`.

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Commit messages:
 - Tweaked use of `register`
 - 8253823: Investigate ways to make handoff-like operation more explicit (foreign-jextract edition)

Changes: https://git.openjdk.java.net/panama-foreign/pull/366/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=366&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253823
  Stats: 5 lines in 5 files changed: 0 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/366.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/366/head:pull/366

PR: https://git.openjdk.java.net/panama-foreign/pull/366