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: nodejs#1580
PR-URL: nodejs#1590
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

Author: indutny

lib/net.js

@@ -201,6 +201,7 @@ function onSocketFinish() {
 
   var req = new ShutdownWrap();
   req.oncomplete = afterShutdown;
+  req.handle = this._handle;
   var err = this._handle.shutdown(req);
 
   if (err)
@@ -627,6 +628,7 @@ Socket.prototype._writeGeneric = function(writev, data, encoding, cb) {
   }
 
   var req = new WriteWrap();
+  req.handle = this._handle;
   req.oncomplete = afterWrite;
   req.async = false;
   var err;