Put a time-to-live on negative binary cache lookups

Negative lookups are purged from the DB after a day, at most once per
day.  However, for non-"have" lookups (e.g. all except "nix-env
-qas"), negative lookups are ignored after one hour.  This is to
ensure that you don't have to wait a day for an operation like
"nix-env -i" to start using new binaries in the cache.

Should probably make this configurable.