RFR: 8269280: (bf) Replace StringBuffer in *Buffer.toString()

[Aleksey Shipilev]

There is a StringBuffer use in X-Buffer template. The extra thread safety is not required there. In post-BiasedLocking times, this is gets worse, as every append is synchronized.

Additional testing:
 - [x] `java/nio/Buffer` tests, verified current `toString` output is unchanged

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Commit messages:
 - 8269280: (bf) Replace StringBuffer in "X-Buffer".toString()

Changes: https://git.openjdk.java.net/jdk/pull/4581/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4581&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269280
  Stats: 654 lines in 9 files changed: 638 ins; 5 del; 11 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4581.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4581/head:pull/4581

PR: https://git.openjdk.java.net/jdk/pull/4581