db: add support for --no-cache

Implement --no-cache. The index is read directly from network and not
cached. This is useful for docker, where you install a set of packages
and directly after purge the cache. (see
https://github.com/gliderlabs/docker-alpine/blob/1fc9e59d1689fc4eaf930ec66389fe58062fccec/builder/scripts/apk-install)

fixes #4905