RFR: 8357671: JFR: Remove JfrTraceIdEpoch synchronizing [v2]

[Markus Grönlund]

> Greetings,
> 
> The JfrTraceIdEpoch::is_synchronizing() method and the field _synchronizing are remnants from the old days when we attempted to force CompilerThreads running in native mode into the correct epoch, which ultimately failed due to races.
> 
> Removing all that old cruft. Also, reuse the existing epoch generation value for the string pool.
> 
> Testing: jdk_jfr
> 
> Thanks
> Markus

Markus Grönlund has updated the pull request incrementally with one additional commit since the last revision:

  extended comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25625/files
  - new: https://git.openjdk.org/jdk/pull/25625/files/990275ad..f66b90ee

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25625&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25625&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/25625.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25625/head:pull/25625

PR: https://git.openjdk.org/jdk/pull/25625