Store prev/current justified root

[djrtwo]

issue

Attestation processing assumes that the previous/current block root will be available in state.latest_block_roots when processing attestations.

attestation.data.justified_block_root == get_block_root(
        state, get_epoch_start_slot(attestation.data.justified_epoch)
)

The call to get_block_root fails if more than SLOTS_PER_HISTORICAL_ROOT have passed since latest justified block. This is currently only about ~13 hours which requires liveness wrt justification in this period for the chain to continue to function. Oh no!

proposed solution

Add previous_justified_root and current_justified_root directly to state to eliminate this lookup to a potentially unavailable slot.

(Thanks @arnetheduck for running your non-justifying chain for 8192 slots!)

[arnetheduck]

It might not necessarily have to be part of the state (duplicate info, kind of).. we can cache it somewhere in the client as well - we already keep a few other book-keeping fields like this around anyway.

[djrtwo]

State transition should be self contained wrt pre_state, block.
I'm not currently aware of any other book keeping fields that are required in the state transition (that said, I expect clients to cache things in various ways as they decide worthwhile).

[arnetheduck]

fair enuff, I was still on a version that required the previous block.

[arnetheduck]

actually, there's nuance here. it doesn't look like the fields are needed to apply a block state transition - they're needed to fill out a block when you're proposing one, no?

[djrtwo]

actually, there's nuance here. it doesn't look like the fields are needed to apply a block state transition - they're needed to fill out a block when you're proposing one, no?

@arnetheduck which fields? access to the root of the justified epoch is needed both when creating an attestation and when validating it.

[arnetheduck]

when validating it.

right, my bad. missed this one.