RFR: 8256256: UL should not use heap allocation for output string [v3]

[Yumin Qi]

> Hi, Please review
>   Unified Logging uses 512 bytes buffer on stack for printing out message, above that will allocate heap for extra space needed. This may potentially may cause a circulation when we log the heap allocation and the log message is over 512. The max logging buffer size now is increased to 4096 and above that, output will be truncated.
> 
> Tests: tier1,tier4

Yumin Qi has updated the pull request incrementally with one additional commit since the last revision:

  Use existing buffer to log the least message when OOM at expanding buffer

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1246/files
  - new: https://git.openjdk.java.net/jdk/pull/1246/files/46b58626..9ee27c40

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1246&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1246&range=01-02

  Stats: 30 lines in 1 file changed: 17 ins; 7 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1246.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1246/head:pull/1246

PR: https://git.openjdk.java.net/jdk/pull/1246