Vacuum: a mode that keeps meta data

[reynir]

I think the current implementation removes old builds and artifacts. This may be undesirable as it breaks albatross-client update - which looks up the hash of the currently running unikernel image. If the build has been purged from builder-web it will not be able to find the build (nor any newer images).

So maybe vacuum needs to arguments: how long to keep artifacts? And how long to keep build information? It could for example be 7 days for artifacts and 7 years for build information.

Care should be taken that removing artifacts only doesn't break builder-web.

[hannesm]

I'm not convinced that we should do this, since it would break the invariant in builder-web that for each build the artifacts are present. Esp. stuff like compare would also not work.

I guess in the presence of vacuum, I'd rather have a "update from " -- so manually providing a job name (to use if the 'find the job from the running unikernel' doesn't work).