RFR: 8294362: UL: Replace the internal usage of manual buffers with stringStream in LogSelection

[Johan Sjölen]

Hi,

This is only a refactoring where the usage of manual character buffers are replaced with output streams, leading to much simpler code.

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Commit messages:
 - Simplify string handling in LogSelection

Changes: https://git.openjdk.org/jdk/pull/10553/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10553&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294362
  Stats: 78 lines in 6 files changed: 2 ins; 44 del; 32 mod
  Patch: https://git.openjdk.org/jdk/pull/10553.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10553/head:pull/10553

PR: https://git.openjdk.org/jdk/pull/10553