setCookie method to suppress enclosing the cookie value field
  in double quotes.

- Avoid module-scope imports.

- Avoid complex shared setup.

- Remove run-as-script cruft.

Removed Zope2's own mkzeoinstance script in favor of the version from the new zope.mkzeoinstance package.

LP #267820

LP #351006

LP #411837

LP #435728

Do this by sharing the thread which processes emails for that directory.

Fixes LP #574286