RFR: 8257234 : Add gz option to SA jmap to write a gzipped heap dump [v6]

[Lin Zang]

> 8257234 : Add gz option to SA jmap to write a gzipped heap dump

Lin Zang has updated the pull request incrementally with one additional commit since the last revision:

  code refine

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1712/files
  - new: https://git.openjdk.java.net/jdk/pull/1712/files/c720aca2..b11434a8

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1712&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1712&range=04-05

  Stats: 20 lines in 3 files changed: 1 ins; 3 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1712.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1712/head:pull/1712

PR: https://git.openjdk.java.net/jdk/pull/1712