RFR: 8252842: Extend jmap to support parallel heap dump

[Chris Plummer]

On Thu, 28 Jan 2021 07:30:29 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>  It is also in my memory that you actually did not have 4 arguments.
> The real incompatibility issue was that the order of arguments was swapped.

Ah, now the fix makes a lot more sense. I was always wondering how changing the order allowed reducing the max args to 3, but apparently the two changes are not related. Also, it seems them that [JDK-8219721](https://bugs.openjdk.java.net/browse/JDK-8219721) does not have a correct analysis of the issue. It says:

> JDK-8215622 increased number of arguments of Attach API [1].
> Thus AttachListener will be waiting the command from client infinitely. 

Perhaps it should be updated to avoid future confusion.

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PR: https://git.openjdk.java.net/jdk/pull/2261