RFR: 8255858: Add debug agent support for storing thread names

[Chris Plummer]

Simple change to add a `name` field to `ThreadNode` to aide when debugging the debug agent. It's off by default. You need to add a `#define DEBUG_THREADNAME` to enable it.

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Commit messages:
 - Add support for tracking a thread's name.

Changes: https://git.openjdk.java.net/jdk/pull/1047/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1047&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255858
  Stats: 19 lines in 1 file changed: 19 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1047.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1047/head:pull/1047

PR: https://git.openjdk.java.net/jdk/pull/1047