RFR: 8269268: JDWP: Properly fix thread lookup assert in findThread() [v2]

[Chris Plummer]

> Re-enable the assert that was disabled (with some overhead) by [JDK-8265683](https://bugs.openjdk.java.net/browse/JDK-8265683). Explanation is in the CR and also in comments included with the changes.
> 
> I tested by running `vmTestbase/nsk/jdb/suspend/suspend001/suspend001.java` and `vmTestbase/nsk/jdb/wherei/wherei001/wherei001.java` 100's of times, and did not see any failures. I also verified the original issue was still reproducible by temporarily not setting `gdata->handlingVMDeath = JNI_TRUE`, which did trigger the assert as expected.

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

  Rename flag and make some minor adjustments.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4580/files
  - new: https://git.openjdk.java.net/jdk/pull/4580/files/7d4a57d3..6e0e54a1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4580&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4580&range=00-01

  Stats: 17 lines in 3 files changed: 5 ins; 6 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4580.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4580/head:pull/4580

PR: https://git.openjdk.java.net/jdk/pull/4580