net: ensure Write/ShutdownWrap references handle

`StreamBase::AfterWrite` is passing handle as an argument to the
`afterWrite` function in net.js. Thus GC should not collect the handle
and the request separately and assume that they are tied together.

With this commit - request will always outlive the StreamBase instance,
helping us survive the GC pass.

Same applies to the ShutdownWrap instances, they should never be
collected after the StreamBase instance.

Fix: https://github.com/iojs/io.js/pull/1580
PR-URL: https://github.com/iojs/io.js/pull/1590


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>