[1.4.X] Fixed #18577 - Clarified middleware initialization.

  define ``__init__`` as requiring any arguments.

* Unlike the ``process_*`` methods which get called once per request,

  ``__init__`` gets called only *once*, when the Web server starts up.

  ``__init__`` gets called only *once*, when the Web server responds to the

  first request.

Marking middleware as unused

~~~~~~~~~~~~~~~~~~~~~~~~~~~~