We can now pass more than one set of `nonce_pks` when creating a CFD.
Each set corresponds to (and is identified by) a different oracle
event, allowing us to construct CFDs which can be settled via oracle
attestation at more than one point in time.
We also remember the CET-event relationship in `CfdTransactions`. This
is needed so that we can select the correct CET when the oracle
attests to the outcome of an event. Additionally, we save the
`nonce_pks` to more easily verify the adaptor signatures.