Fix duplicate requests to provider sites

When the user follows a link from a proxy-by-port EZProxy to another
proxy-by-port EZProxy, we make an additional request to the target site
with no cookies. This yields a redirect to the EZProxy login page. We
use the query parameters to determine the true domain of the site.

Previously, we allowed the redirect to continue, and used the same code we
use to detect EZProxy logins to map the proxied domain to the true domain.
This caused duplicate requests to providers and apparently infinite loops.

We now read the query parameter directly from the redirect to the EZProxy
login page and then cancel the request so that the redirect doesn't get
followed.

The changeset also improves logging of proxy-related information.

See https://forums.zotero.org/discussion/28505/ for further discussion.