SSL.Connection: Added query_write_queue_size().

This provides an easy API to see whether it is meaningful to
call to_write().
SSL.ServerConnection: Fixed nextprotoneg handshake.

The padding wasn't validated, and the wrong value was returned from
handle_handshake().

NB: As this stuff belongs to an obsolete draft, which has been
superceeded by RFC 7301, we probably ought to remove it.