RFR: 8254103: Shenandoah: Move updating thread roots to concurrent phase

[Zhengyu Gu]

Due to non-atomic LRB, there can be from-space oops in thread roots after evacuation.
To enforce to-space invariant, Shenandoah updates thread roots during final reference updating pause, but it can be
done outside pause via thread handshake.

Test:

- [x] hotspot_gc_shenandoah

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

Commit messages:
 - Fix indentations
 - Merge branch 'master' into JDK-8254103-conc-threads-update
 - Update
 - Init update

Changes: https://git.openjdk.java.net/jdk/pull/541/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=541&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254103
  Stats: 48 lines in 4 files changed: 46 ins; 2 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/541.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/541/head:pull/541

PR: https://git.openjdk.java.net/jdk/pull/541