Rethink state pruning

Several problems are linked with the in chain pruning of useless data like old identities. We have to rethink this and find better solutions.

changed milestone to %Horizon

added S-needdiscussion label

For clarification, some automatic actions are better in-chain like cert expiration after two years when not renewed and expiration of membership when received cert count pass below threshold. This makes sure that everyone is treated equally.

But pruning old identities is not subject to the same equality of treatment since it is only for technical reasons (do not have too large storage) and can be delegated to an offchain process.

mentioned in merge request !248 (merged)

mentioned in issue #203 (closed)

To add some context, our fear is that on_initialize of a block exceeds its capacity.

mentioned in issue #72

mentioned in issue #73

removed milestone %Horizon