Fetch/Watch latest versions of select packages

[pombredanne]

I would like to ensure that PurlDB has up-to-date packages for my projects.
For this I would like to somehow register interest for some package version, and purlDB would go fetch newer version of these packages on a regular basis and also would fetch the newer versions of the whole dependencies set of these packages all the way down. 🐢🐢🐢🐢🐢🐢🐢

See also #87

Other things to consider carried over from #64 :

• Trigger mining, matching, scanning of the dependency trees, possibly with priority for runtime dependencies over others
• Trigger mining, matching, scanning for newer versions of packages that were matched
• Trigger mining, matching, scanning for older versions of packages that were matched
• Create a watcher that looks periodically for new versions of a package that was requested, may be by registering interest in this package

[pombredanne]

We have ways to trigger the indexing of a package version range now, but nothing specifically to do anything periodically.
Some inspiration:

• Debian watch files
• Fedora Bohdi

[pombredanne]

Closed in favor of #244 (comment)