inspector: refactor to rename and comment methods

Pure refactor, makes no functional changes but the renaming helped me
see more clearly what the relationship was between methods and
variables.

* Renamed methods to reduce number of slightly different names for the
  same thing ("thread" vs "io thread", etc.).
* Added comments where it was useful to me.

PR-URL: https://github.com/nodejs/node/pull/13321


Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>