RFR: Make use of nanoseconds for GC times [v2]

[Zhengyu Gu]

On Fri, 3 Dec 2021 00:01:56 GMT, David Alvarez <duke at openjdk.java.net> wrote:

>> In multiple places for hotspot management the resolution used for times is milliseconds. With new collectors getting into sub-millisecond pause times, this resolution is not enough.
>>
>> This change moves internal values in LastGcStat to use milliseconds. GcInfo is still reporting the values in milliseconds for compatibility reasons
>
> David Alvarez has refreshed the contents of this pull request, and previous commits have been removed. The incremental views will show differences compared to the previous content of the PR.

I think this patch is not Shenandoah specific, should go to upstream directly.

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PR: https://git.openjdk.java.net/shenandoah/pull/102