[jdk15u-dev] RFR: 8257621: JFR StringPool misses cached items across consecutive recordings

[Ekaterina Vergizova]

I'd like to backport JDK-8257621 to 15u for parity with 11u.
The patch applies almost cleanly except for changes in test/jdk/ProblemList.txt, they are skipped as not applicable to 15u.
Tested with tier1 and jdk/jfr tests.

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Commit messages:
 - Backport 7aac4dc17573b7df2af8aa3d1f01add56b30ad02

Changes: https://git.openjdk.java.net/jdk15u-dev/pull/69/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk15u-dev&pr=69&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257621
  Stats: 222 lines in 13 files changed: 71 ins; 116 del; 35 mod
  Patch: https://git.openjdk.java.net/jdk15u-dev/pull/69.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk15u-dev pull/69/head:pull/69

PR: https://git.openjdk.java.net/jdk15u-dev/pull/69