operational certificate start

[JaredCorduan]

We need to decide how the operational certificate start times are determined, and adjust the spec accordingly.

In particular, is this a value specified in the certificate, or is it determined by the first slot that it is used for a valid block? If we want the certificate to store this value, then we need to add another mapping for this information to the ledger state.

In the design doc, it says that the start time is an epoch number. Should it instead be a KES period (which we can think of as ~1 day)?

Make sure to update the BHEAD rule. It currently has a predicate "hk |-> s_0 \in stpools". This is incorrect either way, as it is the start of the pool registration certificate.