RFR: 8229517: Support for optional asynchronous/buffered logging [v5]

[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:

  Inject the number of dropped messages since last dumpping.
  
  Each LogOutput has an independent counter. The out-of-band message
  "[number] messages dropped..." will be dumped into its corresponding
  LogOutput.

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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/54aadfaf..f453d03f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3135&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3135&range=03-04

  Stats: 95 lines in 8 files changed: 63 ins; 21 del; 11 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