RFR: 8365919: Replace currentTimeMillis with nanoTime in Stresser.java [v4]

[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 with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains six additional commits since the last revision:

 - review
 - Merge branch 'master' into test-nano-time
 - review
 - review
 - Merge branch 'master' into test-nano-time
 - test-nano-time

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/26879/files
  - new: https://git.openjdk.org/jdk/pull/26879/files/10404cea..994e3fe3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=26879&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=26879&range=02-03

  Stats: 1536 lines in 74 files changed: 777 ins; 544 del; 215 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