RFR: 8254103: Shenandoah: Move updating thread roots to concurrent phase [v2]

[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

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Update

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/541/files
  - new: https://git.openjdk.java.net/jdk/pull/541/files/18bfac35..5dd16f59

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=541&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=541&range=00-01

  Stats: 5 lines in 1 file changed: 2 ins; 3 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