Demonstrate time-based slots

[JoshOrndorff]

A non-parachain node may also like to use Nimbus using wall-clock time as the slot trigger. This can easily be added on the runtime side by implementing the SlotBeacon trait for the timestamp pallet. On the client side we will need some work. Probably the best path forward would be to implement Substrate's SlotWorker trait.

[JoshOrndorff]

This will require a little more work on the client side. Currently we have no way of directly enforcing that there is only one block in each slot. If the slots are driven by a block height (either our native height or a backing relay chain's block height), we implicitly have this guarantee because there cannot be two blocks in the same chain at the same height. But if we expend to a non-block-height-based notion of slots, such as wall clock time, then we need to explicitly guarantee that no block is on a lower (or equal) slot than its parent.

[JoshOrndorff]

Actually, ensuring that there are not two blocks in the same slot in the same chain in the runtime should be pretty easy. We can just store the slot number of the last block. Then when executing this block we calculate the current slot number from the slot beacon trait and make sure it is at least as large as the previously stored slot number.

Of course this does nothing about two blocks in the same slot in different chains. That falls under equivocation detection.