RFR: 8317433: Async UL: Only grab lock once when write():ing

[Johan Sjölen]

Hi,

The writer thread used to await for `_data_available` to become true through using the async log lock as a condition variable. When data is available it'd let go of the lock and then call `AsyncLogWriter::write()`, which would immediately try to acquire that same lock. We can remove this second instance by moving the work needed to be done under lock to `run`.

Thank you for considering this.

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Commit messages:
 - Copyright
 - Do not copy the stringStream's underlying buffer
 - Only acquire the lock once for writing

Changes: https://git.openjdk.org/jdk/pull/16030/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16030&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8317433
  Stats: 45 lines in 2 files changed: 18 ins; 23 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/16030.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16030/head:pull/16030

PR: https://git.openjdk.org/jdk/pull/16030