Make sure not to invalidate SSL sessions when closing with https_compatible=True

[njsmith]

Allegedly, if you use OpenSSL and close an SSL connection without sending or receiving a close-notify, then OpenSSL invalidates any negotiated session ticket. If true then this is annoying and bad. Future versions of Python might work around it, but possibly we should work around it first.

I believe this is easy: when we want to do an unclean shutdown, then tell our SSLObject to do a clean shutdown... and then throw away the bytes it gives us, instead of sending them. Gross hack, but it should work.

How do we test this? We already have a bunch of pyopenssl code for testing; maybe it gives some way to tell whether a session is successfully reused?