This significantly speeds up the download-using-manifests
substituter, especially if manifests are very large. For instance,
one "nix-build -A geeqie" operation that updated four packages using
binary patches went from 18.5s to 1.6s. It also significantly
reduces memory use.
The cache is kept in /nix/var/nix/manifests/cache.sqlite. It's
updated automatically when manifests are added to or removed from
/nix/var/nix/manifests. It might be interesting to have nix-pull
store manifests directly in the DB, rather than storing them as
separate flat files, but then we would need a command line interface
to delete manifests from the DB.
