RFR: 8245309: Re-examine use of ThreadLocalCoders in sun.net.www.ParseUtil

[Julia Boes]

Replaced the use of ThreadLocalCoders with regular non-caching CharsetEncoder and added a benchmark to confirm that
there is no performance impact.

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Commit messages:
 - Replace ThreadLocalCoders and add benchmark

Changes: https://git.openjdk.java.net/jdk/pull/170/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=170&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8245309
  Stats: 90 lines in 2 files changed: 81 ins; 2 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/170.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/170/head:pull/170

PR: https://git.openjdk.java.net/jdk/pull/170