stream_base: various improvements

Expose and use in TLSWrap an `v8::External` wrap of the
`StreamBase*` pointer instead of guessing the ancestor C++ class in
`node_wrap.h`.

Make use of `StreamBase::Callback` structure for storing/passing both
callback and context in a single object.

Introduce `GetObject()` for future user-land usage, when a child class
is not going to be inherited from AsyncWrap.

PR-URL: https://github.com/nodejs/node/pull/2351


Reviewed-By: Trevor Norris <trev.norris@gmail.com>