LockoutsConflict in handling VoteStateUpdates with old roots

[carllin]

Problem

In a VoteStateUpdate, when a proposed root is:

1. Not in the SlotHashes history
2. Bigger than existing slots in the VoteState

It will get rejected due to LockoutsConflict

For instance:

Current VoteState {root: 0, 1, 2 ,5, 11, 15}
SlotHashes: {8}
Proposed VoteStateUpdate: {root: 5, 11, 15}

Because the proposed root 5 is not in SlotHashes, we currently leave the root as 0. Then we fail with LockoutsConflict on processing the VoteStateUpdate because 1, 2 should have been included in the VoteStateUpdate and not have been popped off from the current VoteState

Proposed Solution

Set root to largest slot in VoteState less than the proposed root