RFR: 8255955: Shenandoah: Only STW GC should process concurrent roots at pauses

[Zhengyu Gu]

This is a followup of JDK-8255847.

In early versions of Shenandoah, it marked through weak roots when class unloading is disabled. This is no longer the case. 

After moving weak roots (except that JVMTI weak root) processing to concurrent phase for concurrent GC, only STW GCs (degenerated GC and full GC) need to process concurrent weak roots at pause.

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Commit messages:
 - Merge branch 'master' into JDK-8255955-stw-conc-roots
 - 8255955: Shenandoah: Only STW GC should process concurrent roots at pauses
 - Merge branch 'master' into JDK-8255847-root-verifier
 - Remove StrDedup root from concurrent root scanner
 - 8255847: Shenandoah: Shenandoah root verifier's roots_do() should not include weak roots

Changes: https://git.openjdk.java.net/jdk/pull/1081/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1081&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255955
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1081.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1081/head:pull/1081

PR: https://git.openjdk.java.net/jdk/pull/1081