RFR: JDK-8300416 java.security.MessageDigestSpi clone can result in thread-unsafe clones

[Mark Powers]

https://bugs.openjdk.org/browse/JDK-8300416

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Commit messages:
 - first iteration

Changes: https://git.openjdk.org/jdk/pull/12348/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12348&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300416
  Stats: 57 lines in 2 files changed: 52 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/12348.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12348/head:pull/12348

PR: https://git.openjdk.org/jdk/pull/12348