These hashes are useful for OCSP computations, as they are the two
values that are used to uniquely identify the issuer of the given cert in
an OCSP request. Here, they are restricted to SHA1 only, as Boulder
only supports SHA1 for OCSP, as per RFC 5019.
In addition, because the `ID`, `NameID`, `NameHash`, and `KeyHash`
are relatively expensive to compute, introduce a new constructor for
`issuance.Certificate` that computes all four values at startup time and
then simply returns the precomputed values when asked.
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