RFR: 8344328: (dc) DatagramChannelImpl.blockingReceive can now synchronize on packet

[Alan Bateman]

Now that JEP 491 is integrated, we can restore long-standing behavior in the DatagramSocket adaptor to synchronize on the DatagramPacket for the duration of the send/receive.

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Commit messages:
 - Merge branch 'JDK-8344328'
 - More improvements
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/22212/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22212&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344328
  Stats: 39 lines in 1 file changed: 6 ins; 12 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/22212.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22212/head:pull/22212

PR: https://git.openjdk.org/jdk/pull/22212