RFR: Bring genshen up to date with master

[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.

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Commit messages:
 - Merge branch 'master' into github-genshen
 - Merge remote-tracking branch 'upstream/master'
 - 8260571: Add PrintMetaspaceStatistics to print metaspace statistics upon VM exit
 - 8260420: C2 compilation fails with assert(found_sfpt) failed: no node in loop that's not input to safepoint
 - 8257086: Clarify differences between {Float, Double}.equals and ==
 - 8260576: Typo in compiler/runtime/safepoints/TestRegisterRestoring.java
 - 8260864: ProblemList two security/krb5 tests on Linux
 - 8260860: ProblemList tools/jlink/plugins/CompressorPluginTest.java
 - 8213226: [TESTBUG] Reduce the usage of CDSTestUtils.executeAndLog()
 - 8260349: Cannot programmatically retrieve Metaspace max set via JAVA_TOOL_OPTIONS
 - ... and 769 more: https://git.openjdk.java.net/shenandoah/compare/28fc3420...74b47181

Changes: https://git.openjdk.java.net/shenandoah/pull/14/files
 Webrev: https://webrevs.openjdk.java.net/?repo=shenandoah&pr=14&range=00
  Stats: 156854 lines in 3616 files changed: 75939 ins; 48315 del; 32600 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