Slot length changes should probably be ignored by the update system

[deepfire]

Currently slot length appears to be tracked in the update system state:

https://github.com/input-output-hk/cardano-ledger/blob/master/cardano-ledger/src/Cardano/Chain/Update/ProtocolParametersUpdate.hs#L148

..and https://github.com/input-output-hk/cardano-ledger/blob/master/cardano-ledger/src/Cardano/Chain/Update/Validation/Registration.hs#L338

This change in slot duration is replicated across update system state, but is however ignored, as far as I could see (i.e. it stays confined to the US state itself).

@dcoutts suggests we should change things, so this change is instead ignored as soon as the update proposal comes from the wire.

[deepfire]

cc @dnadales

[dnadales]

A possible way of solving this is to just vanish the parameter: ignore the value on decoding, and use a fixed constant on encoding.

However, the important thing is to remove the slot length from any state we maintain. If it's still there in the data structure we read from the old chain format, that's not too important.

This is a nice to have, but not required for mainnet.

[edsko]

Related IntersectMBO/ouroboros-network#282.

[edsko]

Marking this as low priority, because consensus ignores this field entirely anyway.

[edsko]

Related: #494.