RFR: Make use of nanoseconds for GC times

[David Alvarez]

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

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Commit messages:
 - Make use of nanoseconds for GC times

Changes: https://git.openjdk.java.net/shenandoah/pull/102/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=102&range=00
  Stats: 35 lines in 6 files changed: 28 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/102.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/102/head:pull/102

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