RFR: 8365919: Replace currentTimeMillis with nanoTime in Stresser.java [v3]

[Albert Mingkun Yang]

> Mostly a mechanic change from `currentTimeMillis()` to `nanoTime()/1000000`. It also removes `finishTime` to avoid overflowing.
> 
> The change in `iteration()` is to ensure `currentTime` is properly initialized.
> 
> (Was investigating a timeout on Windows-x64, which led me to this code. I think this fix is good enough by its own.)
>  
> Test: tier1-5

Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:

  review

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Changes:
  - all: https://git.openjdk.org/jdk/pull/26879/files
  - new: https://git.openjdk.org/jdk/pull/26879/files/cefb9948..10404cea

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26879&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26879&range=01-02

  Stats: 14 lines in 1 file changed: 1 ins; 1 del; 12 mod
  Patch: https://git.openjdk.org/jdk/pull/26879.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26879/head:pull/26879

PR: https://git.openjdk.org/jdk/pull/26879