Stop reading from object server when client disconnects.

If a client were in the middle of an object GET request and then
disconnected, the proxy would wait a while (default 60s) and then time
out the connection. As part of the teardown for this, the proxy would
attempt to close the connection to the object server, then drain any
associated buffers. However, this didn't work particularly well,
resulting in the proxy reading the entire remainder of the object for
no gain.

Now, the proxy closes the connection hard, by calling .close() on the
underlying socket._socket object. This is different from calling
.close() on a socket._socketobject object, which is what you get back
from socket.socket() and similar methods. Calling .close() on a
socket._socketobject simply decrements a reference counter on the
socket._socket, which has been observed in the past to result in
socket leaks when something holds onto a reference. However, calling
.close() on a socket._socket actually closes the socket regardless of
who else has a reference to it.

I had to delete a test assertion that said the object server never got
SIGPIPE after a GET w/X-Newest. Well, you get a SIGPIPE when you write
to a closed socket, and now the proxy is actually closing the sockets
early, so now you *do* get a SIGPIPE.

closes-bug: 1174660

Note that this will cause a regression on bug 1037337; unfortunately,
the cure is worse than the disease, so out it goes.

Change-Id: I9c7a2e7fdb8b4232e53ea96f86b50e8d34c27221
Merge "Stop reading from object server when client disconnects."