RFR: 8341906: Optimize ClassFile writing BufBuffer

[Shaojin Wen]

We made a lot of improvements to merge writes to BufWriter in #21243, and this PR is about doing more of the same.

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Commit messages:
 - merge write

Changes: https://git.openjdk.org/jdk/pull/21437/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21437&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341906
  Stats: 154 lines in 13 files changed: 43 ins; 35 del; 76 mod
  Patch: https://git.openjdk.org/jdk/pull/21437.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21437/head:pull/21437

PR: https://git.openjdk.org/jdk/pull/21437