buffer: replace deprecated SetWeak usage

Old style SetWeak is now deprecated, and weakness now works like
phantom references. This means we no longer have a reference to the
object in the weak callback. We use a kInternalFields style weak
callback which provides us with the contents of 2 internal fields
where we can squirrel away the native buffer pointer.

We can no longer neuter the buffer in the weak callback, but that
should be unnecessary as the object is going to be GC'd during the
current gc cycle.

PR-URL: https://github.com/nodejs/node/pull/5204


Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: indutny - Fedor Indutny <fedor.indutny@gmail.com>