RFR: 8296089: Remove debug agent code for special handling of Thread.resume() [v2]

[Chris Plummer]

> The debug agent sets a breakpoint in Thread.resume() so it can prevent the debugger from suspending threads while in the resume call:
> 
>              /*
>               * Track the resuming thread by marking it as being within
>               * a resume and by setting up for notification on
>               * a frame pop or exception. We won't allow the debugger
>               * to suspend threads while any thread is within a
>               * call to resume. This (along with the block below)
>               * ensures that when the debugger
>               * suspends a thread it will remain suspended.
>               */
>              trackAppResume(resumer);
> 
> Now that Thread.resume() is unsupported and just throws UnsupportedOperationException, all debug agent code related to this support can be removed. It's at least a couple of hundred lines of code, and with a fair amount of confusing synchronization. It will be nice to see it go.

Chris Plummer has updated the pull request incrementally with one additional commit since the last revision:

  Cleanup some suspect suspendOnStart and toBeResumed checks.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/10922/files
  - new: https://git.openjdk.org/jdk/pull/10922/files/27aefd27..04f7b567

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10922&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10922&range=00-01

  Stats: 18 lines in 1 file changed: 3 ins; 8 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/10922.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10922/head:pull/10922

PR: https://git.openjdk.org/jdk/pull/10922