Before when an AsyncListener object was created and the "create"
callback returned a value, it was necessary to construct a new Object
with the same callbacks but add a place for the new storage value.

Now, instead, a separate storage array is kept on the context which is
used for any return value of the "create" callback. This significantly
reduces the number of Objects that need to be created.

Also added a flags property to the context to quickly check if a
specific callback was available either on the context or on the
AsyncListener instance itself.

Few other minor changes for readability that were difficult to separate
into their own commit.

This has not been optimized yet.

This is a slightly modified revert of bc39bdd.

Getting domains to use AsyncListeners became too much of a challenge
with many edge cases. While this is still a goal, it will have to be
deferred for now until more test coverage can be provided.

BaseObject is a class that just handles the Persistent handle attached
to the class instance.

This also removed WeakObject. Reordering the inheritance chain helps
prevent unneeded calls on instances that don't call MakeCallback.

The domain module has been switched over to use the domain module API as
much as currently possible. There are still some hooks in the
EventEmitter, but hopefully we can remove those in the future.

AsyncListener is a JS API that works in tandem with the AsyncWrap class
to allow the user to be alerted to key events in the life cycle of an
asynchronous event. The AsyncWrap class has its own MakeCallback
implementation that core will be migrated to use, and uses state sharing
techniques to allow quicker communication between JS and C++ whether the
async event callbacks need to be called.