http: Do not buffer writes to a destroyed socket

This fix fills me with sadness.  Here is why:

Prior to v0.10, Node ignored ECONNRESET errors in many situations.
There *are* valid cases in which ECONNRESET should be ignored as a
normal part of the TCP dance, but in many others, it's a very relevant
signal that must be heeded with care.

Exacerbating this problem, if the OutgoingMessage does not have a
req.connection._handle, it assumes that it is in the process of
connecting, and thus buffers writes up in an array.

The problem happens when you reuse a socket between two requests, and it
is destroyed abruptly in between them.  The writes will be buffered,
because the socket has no handle, but it's not ever going to GET a
handle, because it's not connecting, it's destroyed.

The proper fix is to treat ECONNRESET correctly.  However, this is a
behavior/semantics change, and cannot land in a stable branch.  So, the
full-of-sad bandaid fix is to not put data into the output buffer if the
socket is destroyed, and also remove anything that is in the output
buffer when the HTTP request sees that it closes.

/cc @dpacheco @bcantrill @mranney @piscisaureus