async_wrap: correctly pass parent to init callback

Previous logic didn't allow parent to propagate to the init callback
properly. The fix now allows the init callback to be called and receive
the parent if:

- async wrap callbacks are enabled and parent exists
- the init callback has been called on the parent and an init callback
  exists then it will be called regardless of whether async wrap
  callbacks are disabled.

Change the init/pre/post callback checks to see if it has been properly
set. This allows removal of the Environment "using_asyncwrap" variable.

Pass Isolate to a TryCatch instance.

Fixes: https://github.com/nodejs/node/issues/2986
PR-URL: https://github.com/nodejs/node/pull/3216


Reviewed-By: Rod Vagg <rod@vagg.org>