Handle Rekor sharding

[haydentherapper]

/api/v1/log/proof defaults to using the current shard when requesting consistency proofs. This will be an issue once we shard Rekor in production. As the shard changes, the consistency proof can't be calculated, because the tree size will change back down to 1 for the new shard, while the monitor will think the tree has more elements.

We should either:

• Create one monitor per shard
• Make the monitor handle multiple shards. Shard discovery would be needed, and new shards can be found with the /log API.

[haydentherapper]

Beginning work on this to get the monitor working again.

[haydentherapper]

This came up again after yesterday's sharding. Had to delete the artifact for the last successful consistency proof to make it pick up the new log.