RFR: 8256450: Add gz option to jmap to write a gzipped heap dump [v2]

[Chris Plummer]

On Tue, 17 Nov 2020 09:18:20 GMT, Lin Zang <lzang at openjdk.org> wrote:

>> 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:
> 
>   revise the uintx format issue for output message

Are the Attach API changes backwards compatible? You've added a new arg, but that arg could be passed to an older JVM that doesn't support it (I think it just gets ignored), or an older JVM would fail to pass the arg to a new JVM that is expecting it.

Also, please see my comments in [JDK-8256451](https://bugs.openjdk.java.net/browse/JDK-8256451) regarding SA heap dumping support, and how it will lack this ability to compress the heap dump.

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