RFR: 8294361: Cleanup usages of StringBuffer in SQLOutputImpl

[Andrey Turbanov]

There a few unnecessary usages of StringBuffer in SQLOutputImpl class.
All of them create StringBuffer from single character and then concatenate with String. Instead of this, we can concatenate character directly.

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

Commit messages:
 - [PATCH] Cleanup usages of StringBuffer in SQLOutputImpl

Changes: https://git.openjdk.org/jdk/pull/10320/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10320&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294361
  Stats: 36 lines in 1 file changed: 0 ins; 15 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/10320.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10320/head:pull/10320

PR: https://git.openjdk.org/jdk/pull/10320