RFR: 8329766: Serial: Refactor SerialBlockOffsetTable API

[Albert Mingkun Yang]

Simple refactoring towards unifying BOT API between Serial and Parallel.

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Commit messages:
 - s1-bot-api

Changes: https://git.openjdk.org/jdk/pull/18651/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18651&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329766
  Stats: 38 lines in 2 files changed: 11 ins; 25 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18651.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18651/head:pull/18651

PR: https://git.openjdk.org/jdk/pull/18651