Add check that latest index state is not in the future

[michaelpj]

If someone adds a package that is in the future, as happened recently (discussion here: https://github.com/input-output-hk/cardano-haskell-packages/pull/214), then we will accept that fine. The only problem with this is:

1. To use it you have to put your index-state in the future
2. Subsequent people adding packages will have to add timestamps after this future date. This does not play well with all our scripts which assume they can safely put in the current time as the timestamp, they will all not work until the future date passes.

This is a bit of a weird occurrence, but annoying enough that we should probably guard against it.