zoneconcierge: save BTC timestamp proofs upon sealing an epoch

[SebastianElvis]

This is to avoid querying states at a prior height, which might not be compatible with pruning.

• hook upon epoch is sealed
• save proof that CZ header is in an epoch in each epoch's chain info
• new KV store where key is sealed epoch and value is the proofs
• generate and save proof that an epoch is sealed upon sealing this epoch