handle: Don't use proxy cache for downloads of metalink/mirrorlist
handle: Don't use proxy cache for downloads of metalink/mirrorlist

Add download_non_cached_url() function which is used for download of
metalink and mirrorlist. This function sets no_cache flag for
download target.

This commit is related to previous commits which deal with
proxy data caching.

rpm-software-management/librepo,rpm-software-management/librepo,Tojaj/librepo,Tojaj/librepo,rpm-software-management/librepo,Tojaj/librepo