RFR: 8256450: Add gz option to jmap to write a gzipped heap dump [v4]

[Lin Zang]

> This PR add "gz" option to jmap -dump command to support generate gzipped heap dump.
> 
> example:
>       jmap -dump:live,gz=1,file=dump.gz <pid>

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

  fix indentation issue of BasicJmapTest.java

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1251/files
  - new: https://git.openjdk.java.net/jdk/pull/1251/files/1358e45f..f0bd8d81

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1251&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1251&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1251.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1251/head:pull/1251

PR: https://git.openjdk.java.net/jdk/pull/1251