Allow block and attestation production at past slots

## Description

It might be useful for the `BeaconChain` to allow for producing blocks/attestations at previous slots (e.g., late message from a validator).

## Present Behaviour

Attestations and blocks are only produced at the current slot.

## Expected Behaviour

Go back and attempt to load a state from a previous slot and produce based upon it.

## Steps to resolve

NA
