RFR: 8223312: Utilize handshakes instead of is_thread_fully_suspended

[Robbin Ehn]

The main point of this change-set is to make it easier to implement S/R on top of handshakes.
Which is a prerequisite for removing _suspend_flag (which duplicates the handshake functionality).
But we also remove some complicated S/R methods.

We basically just put in everything in the handshake closure, so the diff just looks much worse than what it is.

TraceSuspendDebugBits have an ifdef, but in both cases it now just returns.
But I was unsure if I should remove now or when is_ext_suspend_completed() is removed.

Passes multiple t1-5 runs, locally it passes many jck:vm/nsk_jvmti/nsk_jdi/jdk-jdi runs.

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

Commit messages:
 - Utilize handshakes instead of is_thread_fully_suspended

Changes: https://git.openjdk.java.net/jdk/pull/729/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=729&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8223312
  Stats: 480 lines in 6 files changed: 158 ins; 266 del; 56 mod
  Patch: https://git.openjdk.java.net/jdk/pull/729.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/729/head:pull/729

PR: https://git.openjdk.java.net/jdk/pull/729