RFR: 8365919: Replace currentTimeMillis with nanoTime in Stresser.java

[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

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Commit messages:
 - test-nano-time

Changes: https://git.openjdk.org/jdk/pull/26879/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26879&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365919
  Stats: 23 lines in 1 file changed: 5 ins; 6 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