#79 Feature Request: mark cache entry for expiry

[Dave Abrahams]

When building feeds from source, it's common to have several feeds that need to be constructed from the same build results. The way I handle that is to create a -preinstall feed that contains the build results, and then use that in the building of the several aforementioned feeds. However, once that build is done, the -preinstall feed is just taking up space on the disk. It would be nice to be able to say to ZI, "hey, if nobody's depending on this feed, you can delete it in an hour or so."