RFR: Merge shenandoah:genshen [v2]

[earthling-amzn]

> Brought mainline (master) commits in a series of smaller merges to make the conflicts more manageable. The refactoring of concurrent mark and the control thread required some non-trivial conflict resolution. I've tested `satb` mode and `generational` mode on the dacapo suite.

earthling-amzn has updated the pull request incrementally with one additional commit since the last revision:

  Correct include

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Changes:
  - all: https://git.openjdk.java.net/shenandoah/pull/14/files
  - new: https://git.openjdk.java.net/shenandoah/pull/14/files/74b47181..4809d472

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=14&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=14&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/shenandoah/pull/14.diff
  Fetch: git fetch https://git.openjdk.java.net/shenandoah pull/14/head:pull/14

PR: https://git.openjdk.java.net/shenandoah/pull/14