bpo-24334: Cleanup SSLSocket (#5252)

* The SSLSocket is no longer implemented on top of SSLObject to
  avoid an extra level of indirection.
* Owner and session are now handled in the internal constructor.
* _ssl._SSLSocket now uses the same method names as SSLSocket and
  SSLObject.
* Channel binding type check is now handled in C code. Channel binding
  is always available.

The patch also changes the signature of SSLObject.__init__(). In my
opinion it's fine. A SSLObject is not a user-constructable object.
SSLContext.wrap_bio() is the only valid factory.