RFR: 8258956: Memory Leak in StringCoding on ThreadLocal resultCached StringCoding.Result

[Naoto Sato]

Please review the fix to this issue. Unused thread local StringCoding.Result is now wrapped with SoftReference, which can be GC'ed as needed. I confirmed it worked as expected with a test case (https://github.com/openjdk/jdk/blob/8b4e1e1cf8f5d753ed901406f73d67b21557fddb/test/jdk/java/lang/StringCoding/ResultCachedGCTest.java), but decided not to include it in this PR. This is because I purposefully chose the size of the byte array and max heap size which is fragile, and could become noise in a regression test run.

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Commit messages:
 - Removed the test case.
 - 8258956: Memory Leak in StringCoding on ThreadLocal resultCached StringCoding.Result

Changes: https://git.openjdk.java.net/jdk/pull/2064/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2064&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258956
  Stats: 18 lines in 1 file changed: 9 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2064.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2064/head:pull/2064

PR: https://git.openjdk.java.net/jdk/pull/2064