Ensure only the first V + 1 blocks from a given author at a slot are accepted by the runtime

[Sophia-Gold]

From #2476:

What I'm imagining is an extension to the logic of pallet-aura-ext which does these things in on_initialize:

• Track the slot of the current in a storage item, as well as how many blocks have been authored at that slot. (Slot, u32)
• if the slot is the same as the previous value of the storage item, the counter is incremented.
• if the slot is less than the previous value, then panic
• if the slot is greater than the previous value, then update the storage item to (new_slot, 1).
• To handle the case where the storage item was not present directly after an upgrade, set the value to (current_slot, 1).

Along with a ConsensusHook implementation like this (living in the pallet-aura-ext crate):

struct FixedVelocityConsensusHook<const N: u32>;

• If counter > N + 1, panic.

[rphmeier]

We need a second constant C which is the maximum capacity, actually. so struct FixedVelocityConsensusHook<T: pallet_aura::Config, V, C> where both velocity & capacity are clamped to 1

[slumber]

#2551