RFR: 8229517: Support for optional asynchronous/buffered logging [v21]

[Xin Liu]

> This patch provides a buffer to store asynchrounous messages and flush them to
> underlying files periodically.

Xin Liu has updated the pull request incrementally with two additional commits since the last revision:

 - Remove AsyncLogWriter::flush() from os::shutdown().
   
   When jvm aborts, it won't invoke AsyncLogWriter::flush() and guarantee
   not to hinder the aborting process.
   
   Run all unified logging tests in async mode.
 - Biased to the thread which is taking buffer lock.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3135/files
  - new: https://git.openjdk.java.net/jdk/pull/3135/files/9800a8cc..2ffc33fe

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3135&range=20
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3135&range=19-20

  Stats: 32 lines in 5 files changed: 7 ins; 15 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3135.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3135/head:pull/3135

PR: https://git.openjdk.java.net/jdk/pull/3135