copy compute-ranking.go to dcs-compute-ranking, make it d/l, use gzip

Instead of relying on a Debian mirror, switch to downloading the
necessary files on demand (more useful for the new architecture).

Also, mirrors don’t have the bz2 files anymore, so switch to gzip.
add service + timer files to update the ranking database hourly
dcs-package-importer: add /listpkgs endpoint
dcs-package-importer: create directory if it does not yet exist
dcs-package-importer: run on as many cores as we have available
add quick'n'dirty utility to reshard a setup
gofmt read.go
properly report which index file is corrupt

This is important since we now deal with thousands of index files.
use a json decoder instead of buffering the result in memory
dcs-package-importer: don’t add a leading / to the index
work around index bug by removing leading slashes until the index is rebuilt
use a json.Encoder() instead of marshalling + writing
gofmt index-backend.go
use a json.NewDecoder instead of buffering data in memory
copy source-backend to dcs-source-backend

I’ll modify it and figured it’s a good time to give it the proper name.
dcs-compute-ranking: properly overwrite old entries

(previously, the database would fill up indefinitely)

What’s still left to do is expiring old packages that were removed from
Debian.
dcs-rs-deploy: use http.debian.net instead of the rackspace mirror

The latter is sometimes out of date or just down.
dcs-rs-cleanup: detach block storage before deleting servers

I was advised by Rackspace support to do it this way rather than just
deleting servers.