wire: Zero out secrets signature for commitment hash

[jrick]

The signature changes depending on the published previous RS message hashes, and therefore cannot be used to create the commitment hash covering the actual secrets being published by the message.

[jrick]

Alternatively we could move this method completely outside of the wire package. It could reasonably live as a function in the mixing module instead.

[davecgh]

I'll go ahead and approve and merge it since the change is needed anyway.

I'm fine with it living in the mixing module instead too. I think I've mentioned before that I generally think wire ideally should just be handling the serialization/deserialization bits anyway and all types of commitments and the like handled elsewhere, but in an effort to get the decentralized mixing landed, it's fine to have things not be in the most ideal locations for now.