RFR: 8294075: gtest/AsyncLogGtest crashes with SEGV

[Xin Liu]

In order to test "drop messages scenario", I replace the serving buffers with shrunk buffers(1k).
This is done by RAII class 'AsyncLogWriter::BufferUpdater'. There is a race condition between test and AsyncLog thread. 
The race condition exhibits when AsyncLog thread is still processing `buffer_staging` but test thread deletes it in dtor of BufferUpdater. 

This patch issues AsyncLogWriter::flush() before dtor of BufferUpdater. buffers are reverted only when pending messages have all been handled.

I test TEST="jtreg:hotspot/jtreg/gtest/AsyncLogGtest.java" 100 times and never see race condition.

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Commit messages:
 - 8294075: gtest/AsyncLogGtest crashes with SEGV

Changes: https://git.openjdk.org/jdk/pull/10367/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10367&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294075
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/10367.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10367/head:pull/10367

PR: https://git.openjdk.org/jdk/pull/10367