RFR: 8257234 : Add gz option to SA jmap to write a gzipped heap dump [v4]

[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:

  refine segment heap dump

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1712/files
  - new: https://git.openjdk.java.net/jdk/pull/1712/files/ef353ac3..e7b53589

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1712&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1712&range=02-03

  Stats: 193 lines in 4 files changed: 47 ins; 50 del; 96 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