Backport removal of experimental support for configuring the Twisted HTTP server as an alternative to ``ZServer``.

o Added "sane" registration of it during initialization of Five, instead
  of registering it via import side effects.

Clearing frees resources, closing also sends out the end-request event. Clones
of the current request need to be cleared, only the actual current request
needs to be closed. Fixes LP #414757.

- LP #418454: FTP server did not work with Python 2.6.X