RFR: 8274898: Cleanup usages of StringBuffer in jdk tools modules

[Andrey Turbanov]

StringBuffer is a legacy synchronized class. StringBuilder is a direct replacement to StringBuffer which generally have better performance

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Commit messages:
 - [PATCH] Cleanup usages of StringBuffer in jdk tools modules

Changes: https://git.openjdk.java.net/jdk/pull/5433/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5433&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274898
  Stats: 8 lines in 3 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5433.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5433/head:pull/5433

PR: https://git.openjdk.java.net/jdk/pull/5433