* Handle correctly CancelledError: just exit
* On error, log the exception and exit

Don't try to close the event loop, it is probably running and so it cannot be
closed.

Set the _read_fut attribute to None after cancelling it.

This change should fix a race condition with
_ProactorWritePipeTransport._pipe_closed().

Check if the _sock attribute is None to check if the transport is closed.

The new SSL implementation is based on the new ssl.MemoryBIO which is only
available on Python 3.5. On Python 3.4 and older, the legacy SSL implementation
(using SSL_write, SSL_read, etc.) is used. The proactor event loop only
supports the new implementation.

The new asyncio.sslproto module adds _SSLPipe, SSLProtocol and
_SSLProtocolTransport classes. _SSLPipe allows to "wrap" or "unwrap" a socket
(switch between cleartext and SSL/TLS).

Patch written by Antoine Pitrou. sslproto.py is based on gruvi/ssl.py of the
gruvi project written by Geert Jansen.

This change adds SSL support to ProactorEventLoop on Python 3.5 and newer!

It becomes also possible to implement STARTTTLS: switch a cleartext socket to
SSL.

Close the IocpProactor before closing the event loop. IocpProactor.close() can
call loop.call_soon(), which is forbidden when the event loop is closed.

* Yes, futures errors (Error, CancelledError, TimeoutError, ...) are aliases of
  concurrent.futures errors
* InvalidStateError: the state is already logged in the message when the
  exception is raised
* call_exception_handler() now makes possible to decide how to handle
  exceptions
* Add a docstring to _UnixDefaultEventLoopPolicy

methods of BaseEventLoop now use the identifier of the current thread to ensure
that they are called from the thread running the event loop.

Before, the get_event_loop() method was used to check the thread, and no
exception was raised when the thread had no event loop. Now the methods always
raise an exception in debug mode when called from the wrong thread. It should
help to notice misusage of the API.