In `getAuthorizations`, we had a single loop to both select the freshest authz and
fetch challenges corresponding to authzs. This meant that in some cases, we
would fetch challenges only to throw them away. Since each challenge fetch is a
DB round trip, this would cause extreme slowness when called for domains that
have a large number of authorizations.
This change splits that into two loops: One to select the freshest authzs, and
another to fetch challenges for those authzs.
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