src: refactor tracing agent code

Avoid unnecessary operations, add a bit of documentation,
and turn the `ClientHandle` smart pointer alias into a real
class.

This should allow de-coupling the unnecessary combination of
a single specific `file_writer` from `Agent`.

PR-URL: https://github.com/nodejs/node/pull/21867


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Eugene Ostroukhov <eostroukhov@google.com>
Reviewed-By: Ali Ijaz Sheikh <ofrobots@google.com>