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

[Paul Hohensee]

On Fri, 3 Dec 2021 00:01:56 GMT, David Alvarez <duke at openjdk.org> 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. The pull request contains one new commit since the last revision:
> 
>   Make use of nanoseconds for GC times

This PR should be against jdk tip, not shenandoah.

In timer.cpp, use "counter_to_seconds(counter) * MILLIUNITS" instead of "counter_to_seconds(counter) * 1000.0".

In management.cpp, use NANOUNITS instead of (double)1000000000.0 and MILLIUNITS instead of (double)1000.0.

Otherwise, lgtm.

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Changes requested by phh (no project role).

PR: https://git.openjdk.org/shenandoah/pull/102