RFR: 8258431: Provide a JFR event with live set size estimate [v6]

[Jaroslav Bachorik]

On Mon, 1 Mar 2021 15:24:48 GMT, Jaroslav Bachorik <jbachorik at openjdk.org> wrote:

>> Another general comment about Shenandoah. It would seem easier to piggyback liveness summarization on region iteration that heuristics does at the end of mark anyway. See `ShenandoahHeuristics::choose_collection_set`. I can do that when you are done with your changes, or try it yourself.
>
> I have addressed comments with trivial fixes. 
> Will take a look at the remainder of more complex ones next.

Going over my initial changes I am starting to doubt the usefulness of defaulting to `used` value when `live` estimate is not available. It seems to be giving false information - perhaps it would be ok to return `0` as an invalid value to indicate that that particular information is not available and it would be up to the event consumer to fall back to using the `used` value or deal with the missing value by some other means.

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PR: https://git.openjdk.java.net/jdk/pull/2579