RFR: Only scan dirty cards, not entire regions, in update refs remset scan.

[Bernd Mathiske]

So far in update refs, when iterating over regions to process the remset, we scan an entire region for references into young gen if we find any dirty card in it. With this update, we only process dirty cards in each region and leave out clean ones. To do so, we refactor the scan code from the marking phase into a method that is then reused for update refs, passing down a different closure. The JVM switch `ShenandoahUseSimpleCardScanning` provides a means to fall back on the previous code.

(In the future, we may want to replace region stealing with card stealing.)

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Commit messages:
 - Only scan dirty cards, not entire regions, in remembered set scanning in update refs.

Changes: https://git.openjdk.java.net/shenandoah/pull/22/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=22&range=00
  Stats: 78 lines in 4 files changed: 30 ins; 26 del; 22 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/22.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/22/head:pull/22

PR: https://git.openjdk.java.net/shenandoah/pull/22