Crude fix for issue #855: automatically initiate SSL handshake in SSLIOStream.
Perhaps this should be exposed as an option instead?
Route all callback error logging in IOLoop through handle_callback_exception.
Clear the IOStream buffers on close even when there is no close callback.

Closes #828. (again)
RequestHandler sets its close callback on the HTTPConnection, not the IOStream.

Fixes a bug in which close callbacks would never be called for subsequent
requests on a reused connection.
Merge remote-tracking branch 'pitrou/issue855-ssl-auto-handshake'
Less crude fix for #855: only do anything for already-connected sockets.