epoch trans at end of epoch and fix finality conditions

[djrtwo]

• moves epoch transition to last slot in epoch (addresses Put per-slot processing after per-epoch processing #267 (comment))
• fixes Finalization rule 3 would return True in no case #410 finality conditions
• fixes shard_committees_at_slots missing when processing previous epoch attestations #352 and get_shard_committees_at_slot out of range #409 (shard committees for prev epoch still available when processing attestations)
• includes make epochs first class citizens #499 which converted a lot of the slot logic to be epoch based instead of slot based.

[vbuterin]

Here's a PR to your PR 🤣

#495

The logic that you actually changes all looks good, it's more about a couple other things that still need to be changed.

[djrtwo]

Thanks! Merged the PR into my PR

[vbuterin]

Looks good to me at the moment. Maybe after merging this one and the light client one we can do a final pass on the epoch transition logic. I think for cleanliness reasons it may be a good idea to remove use of state.slot entirely (in favor of current/next/previous epoch start slot) but best do that after these two PRs as they both modify the epoch transition section quite a bit.

[djrtwo]

• handled merge conflict
• modified where current_epoch_calculation_slot is being updated to be the start of the next epoch when it is updated.
• updating latest_index_roots for the next_epoch instead of the current. using this root in the seed generation for the shuffling starting at next epoch
• created a function generate_seed to handle the complexity of the seed generation. Also handles the edge case when slot < SEED_LOOKAHEAD to prevent underflow
• set latest_index_roots for genesis slot/epoch
• use generate_seed to properly embed the active index root in the initial shuffling seed

Did not yet handle the next_ functionality to guarantee 1 epoch lookahead.

I'm starting to think that a lot of these vars and state attributes should just reference epochs. We are doing a lot of error prone math everywhere to get the start of epochs and convert slots to epochs. And we inconsistently use epochs vs slots

[djrtwo]

I'm working on a PR to this PR that makes epochs a first class citizen. Seeing if it cleans things up.

edit: PR here #499

[djrtwo]

Genesis slot changes look good to me @vbuterin

[vbuterin]

Good!

[hwwhww]

I think #505 could close #224.

[ChihChengLiang]

Note that slot_to_epoch use EPOCH_LENGTH that is defined in the next section. So in the chain config, this will look like:

genesis_slot = SlotNumber(2**19)
epoch_length = 2**6
SERENITY_CONFIG = BeaconConfig(
    ...
    GENESIS_SLOT=genesis_slot,
    GENESIS_EPOCH=slot_to_epoch(genesis_slot, epoch_length),
    ...
    EPOCH_LENGTH=epoch_length
    ...
)

[djrtwo]

Not the prettiest. I think we want to move to #504 which will use chain start time to seed the initial state rather than config.