RFR: 8274879: Replace uses of StringBuffer with StringBuilder within java.base classes [v2]

[Andrey Turbanov]

> StringBuffer is a legacy synchronized class. There are more modern alternatives which perform better:
> 1. Plain String concatenation should be preferred
> 2. StringBuilder is a direct replacement to StringBuffer which generally have better performance
> 
> In [JDK-8264029](https://bugs.openjdk.java.net/browse/JDK-8264029)  I migrated only usages which were automatically detected by IDEA. It turns out there are more usages which can be migrated.

Andrey Turbanov has updated the pull request incrementally with one additional commit since the last revision:

  8274879: Replace uses of StringBuffer with StringBuilder within java.base classes
  revert changes in spec to avoid CSR

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/5432/files
  - new: https://git.openjdk.java.net/jdk/pull/5432/files/14005d1d..c8d68c2a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=5432&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=5432&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5432.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5432/head:pull/5432

PR: https://git.openjdk.java.net/jdk/pull/5432