RFR: Fix verifier handling of weak references when scanning the remembered set

[William Kemper]

In some use cases, `ShenandoahVerifier` is required to _ignore_ weak references (because they will be processed concurrently). Because of the way the remembered set scan handles oops, it must use an `OopIterateClosure` (not a bare `OopClosure`). To make use of the `OopClosure` passed to the remembered set scanner, it was wrapped in an "adapter" that sheared off the `ShenandoahIgnoreReferenceDiscoverer` configured on the passed in closure. This change removes the need to use such an adapter during the verifier's root scan.

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Commit messages:
 - Use more descriptive method names
 - Fix verifier handling of weak references when scanning the remembered set

Changes: https://git.openjdk.java.net/shenandoah/pull/41/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=41&range=00
  Stats: 13 lines in 4 files changed: 5 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/41.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/41/head:pull/41

PR: https://git.openjdk.java.net/shenandoah/pull/41