RFR: 8308633: Increase precision of timestamps in g1 log

[Thomas Schatzl]

Hi all,

  please review this improvement adding a digit to log timestamp to improve output. On modern machines, for some pauses, this can lead to sub-phase timings not adding up, and during development, extra work to evaluate different options.
Also that rounding error starts to matter for e.g. pauses < a few ms. 

I am aware that there is a duplication of the timestamp format in the files, and this change did not improve that as this would be a different issue. Not sure where to put such a global constant though, any suggestions?

Testing: gha, looking at log files

Thanks,
  Thomas

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Commit messages:
 - initial version, add digit to timestamps in logs

Changes: https://git.openjdk.org/jdk/pull/14594/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14594&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8308633
  Stats: 8 lines in 3 files changed: 3 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/14594.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14594/head:pull/14594

PR: https://git.openjdk.org/jdk/pull/14594