RFR: 8229517: Support for optional asynchronous/buffered logging [v20]

[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 one additional commit since the last revision:

  flush() waits until all pending logging IO operations are done.
  
  This patch support all gtests in async mode.
  make test TEST="gtest:all" TEST_OPTS='VM_OPTIONS=-Xlog:async'

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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/cad42d79..9800a8cc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3135&range=19
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3135&range=18-19

  Stats: 24 lines in 6 files changed: 15 ins; 7 del; 2 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