RFR: 8325517: Shenandoah: Reduce unnecessary includes from shenandoahControlThread.cpp [v2]

[William Kemper]

> The control thread used to run much more of the cycle directly. This code was all factored out into different classes, but many of the vestigial headers remained. Removing these improves compilation times and makes maintenance easier.

William Kemper has updated the pull request incrementally with two additional commits since the last revision:

 - Even more include clean up
 - Move StringDedupeMode into the only header that needs it

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17778/files
  - new: https://git.openjdk.org/jdk/pull/17778/files/fd24ad00..5212e221

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17778&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17778&range=00-01

  Stats: 26 lines in 5 files changed: 10 ins; 13 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17778.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17778/head:pull/17778

PR: https://git.openjdk.org/jdk/pull/17778