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.

Create a HandleScope before calling the Environment::GetCurrent() that
takes a v8::Isolate* as an argument because it creates a handle with
the call to v8::Isolate::CurrentContext().

Making WeakObject inherit from AsyncWrap allows us to peak into almost
all the MakeCallback calls in Node internals.

Going back to the original names of Wrap/Unwrap now that most all the
class members that duplicate the name and functionality have been
removed.

Switch out to use UnwrapObject from util.h.

Remove the NODE_{WRAP,UNWRAP} macros and instead use template functions.

This addresses a current shortcoming of the V8 SetNamedPropertyHandler
function.

It does not provide a way to intercept Object.defineProperty(..) calls.
As a result, these properties are not copied onto the contextified
sandbox when a new global property is added via either a function
declaration or a Object.defineProperty(global, ...) call.

Note that any function declarations or Object.defineProperty() globals
that are created asynchronously (in a setTimeout, callback, etc.) will
happen AFTER the call to copy properties, and thus not be caught.

The way to properly fix this is to add some sort of a
Object::SetNamedDefinePropertyHandler() function that takes a callback,
which receives the property name and property descriptor as arguments.

Luckily, such situations are rare, and asynchronously-added globals
weren't supported by Node's VM module until 0.12 anyway.  But, this
should be fixed properly in V8, and this copy function should be removed
once there is a better way.

Fix #6416

Drop the ObjectWrap dependency in favor of an internal WeakObject class.

Let's us stop worrying about API and ABI compatibility when making
changes to the way node.js deals with weakly persistent handles
internally.

Apparently, context->Global() won't be destroyed if the context itself
isn't marked as weak and independent.

Also, the weakness flag should be cleared once the weak callback is
executed, otherwise we'll get crashes in Debug builds.

fix #6115 and #6201