* SC-XX: Require distributionPoint in sharded CRLs

The language in RFC 5280 regarding the interaction between the
distributionPoint field of the Issuing Distribution Point CRL extension
and the existence of sharded CRLs has led to significant debate on
interpretation, and appears to contradict X.509.

To protect against replacement attacks, make it explicitly clear that
the Issuing Distribution Point extension and distributionPoint field are
required for sharded or partitioned CRLs.

* Remind readers that the IDP must be critical

* Change effective date to Jan 15

* Change effective date in Section 1.2 table, too