node: fn to add undefined prototype properties

Add convenience function to add undefined properties to a
FunctionTemplate's prototype. This is useful when properties will be set
in the future on the instantiated object, without V8 changing the object
map.
dgram: SendWrap now has FunctionTemplate

In order for SendWrap to have access to the additional methods in
AsyncWrap, the request object needs to be an instance of a
FunctionTemplate.