RFR: 8276904: Optional.toString() is unnecessarily expensive

[Eamonn McManus]

Use string concatenation instead of `String.format`.

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Commit messages:
 - 8276904: Optional.toString() is unnecessarily expensive

Changes: https://git.openjdk.java.net/jdk/pull/6337/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6337&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8276904
  Stats: 4 lines in 4 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6337.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6337/head:pull/6337

PR: https://git.openjdk.java.net/jdk/pull/6337