RFR: Add parallel worker timings for remembered set scan

[earthling-amzn]

### Description
Followed the existing pattern to add parallel working timings for remembered set scan.

### Testing
Ran Dacapo, looks like this in the logs:
[157.867s][202297][info] Concurrent Reset                  94013 us
[157.867s][202297][info] Pause Init Mark (G)              579650 us
[157.867s][202297][info] Pause Init Mark (N)              579456 us
[157.867s][202297][info]   Scan Remembered Set            483166 us, parallelism: 3.91x
[157.867s][202297][info]     RS: <total>                 1891580 us
[157.867s][202297][info]     RS: Scan Clusters           1891580 us, workers (us): 481972, 468615, 457912, 483081, 
[157.867s][202297][info]   Update Region States               78 us
[157.867s][202297][info] Concurrent Mark Roots             18932 us, parallelism: 1.98x

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Commit messages:
 - Add parallel worker timings for remembered set scan

Changes: https://git.openjdk.java.net/shenandoah/pull/27/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=27&range=00
  Stats: 10 lines in 4 files changed: 6 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/27.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/27/head:pull/27

PR: https://git.openjdk.java.net/shenandoah/pull/27