RFR: 8262486: Merge trivial JDWP agent changes from the loom repo to the jdk repo

[Chris Plummer]

In the loom repo there are a number of trivial changes to the JDWP agent that are not loom specific. Moving them to the jdk repo will make future merges simpler, along with making it easier to get a diff of the important differences between the loom and jdk repos, without the clutter of these trivial changes. The changes I'd like to merge are:

1. Very minor code refactoring.
2. Comment additions, fixes, and improvements.
3. Minor indentation and whitespace changes.
4. Added a trivial assert

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Commit messages:
 - Minor jdwp agent edits from loom project.

Changes: https://git.openjdk.java.net/jdk/pull/2815/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2815&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262486
  Stats: 79 lines in 7 files changed: 37 ins; 4 del; 38 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2815.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2815/head:pull/2815

PR: https://git.openjdk.java.net/jdk/pull/2815