consensus: race condition receive epoch end proposal before base layer scanner is at tip

[sdbondi]

A node may receive an epoch end block before it has scanned to the tip.

This results in leader failures since the node decides not to vote for it.

We need to ensure that we have scanned to the tip before processing the block.

Possible solutions:

• park the block until we scan to the epoch
• wait until block height is height(epoch) + 1 before proposing epoch end to allow enough time to avoid the race condition (node will still accept the block if scanned to height(epoch))
• other suggestions?