RFR: 8258431: Provide a JFR event with live set size estimate [v2]

[Jaroslav Bachorik]

On Mon, 1 Mar 2021 14:17:17 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> The change also misses liveness update after G1 Full GC: it should at least reset the internal liveness counter to 0 so that `used()` is used.
>> I think there is the same issue for Parallel Full GC. Serial seems to be handled.
>
> 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.

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