Add consistent log labels for signer_signature_hash

[hstove]

When logging, we aren't totally consistent when doing labeled logs for signer_signature_hash. For example, sometimes we use signer_sighash, sometimes signer_signature_hash, etc. This makes it harder to search for all logs related to a specific signer signature hash, because you can't just do a log query like signer_signature_hash: blah.

It would be great to make this more consistent.