Previous crosslink becomes outdated after epoch transition

[mkalinin]

Issue

After transition to a 3rd epoch crosslink of an attestation created in the last slot of 2nd epoch becomes outdated.

Steps

The steps are taken from simulation with 4 shards and 4 slots in the epoch:

Epoch 0:
  s3: create att.s3.shard_3.previous_crosslink_epoch_0

Epoch 1:
  s4: transition to epoch_1 -> shard_3.latest_crosslink_epoch = 0 (not changed)
  s4: include att.s3.shard_3.previous_crosslink_epoch_0 into pending state 
      (Case 1: latest crosslink matches previous crosslink)

  s7: create att.s7.shard_3.previous_crosslink_epoch_0

Epoch 2:
  s8: transition to epoch_2 -> shard_3.latest_crosslink_epoch = slot_to_epoch(s3) = 1
  s8: include att.s7.shard_3.previous_crosslink_epoch_0 into pending state 
      (Case 2: latest crosslink matches current crosslink)

  s11: create att.s11.shard_3.previous_crosslink_epoch_1

Epoch 3:
  s12: transition to epoch_3 -> shard_3.latest_crosslink_epoch = slot_to_epoch(s11) = 2
  s12: att.s11.shard_3.previous_crosslink_epoch_1 becomes invalid
       (neither from two cases matches)

[mkalinin]

Looks like there is another potential issue around get_winning_root_and_participants.

Assuming there are two sibling epochs: e1 and e2. Considering the case when attestations att.e1.shard_n and att.e2.shard_n have been added to the state during epoch e2.

In that case get_winning_root_and_participants will have to deal with two attestations for shard_n during e2 -> e3 transition where crosslink_data_root of att.e1.shard_n could potentially win.

[hwwhww]

closing via #874