Use pay-to-contract-hashes

[dexX7]

Instead of randomly generating keys, or using deterministic key generation, use pay-to-contract-hashes.

This may simplify the communication and might get rid of having multiple keys complely, if the input transaction is used to seed the hash.

References:

• https://blockstream.com/sidechains.pdf
• http://arxiv.org/pdf/1212.3257v1.pdf