RFR: 8289385: Cleanup redundant synchronization in Http2ClientImpl

[Andrey Turbanov]

Http2ClientImpl.stopping and Http2ClientImpl.failures are always accessed under synchronized(this).
So we can remove 'volatile' modifier from 'stopping'. And remove 'synchronizedSet' wrapper from 'failures'

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Commit messages:
 - [PATCH] Cleanup redundant synchronization in Http2ClientImpl

Changes: https://git.openjdk.org/jdk/pull/9311/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9311&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8289385
  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9311.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9311/head:pull/9311

PR: https://git.openjdk.org/jdk/pull/9311