src: make IsolateData creation explicit

Make it easier to reason about the lifetime and the ownership of the
IsolateData instance by making its creation explicit and by removing
reference counting logic.

The creator of the Environment is now responsible for passing in the
IsolateData instance and for keeping it alive as long as the Environment
is alive.

PR-URL: https://github.com/nodejs/node/pull/7082


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>