Mention interaction of DCs and tickets

[siyengar]

There are a few interactions that could use security considerations text:

1. Some clients re-validate the certificate chain during resumptions. We should mention that you might want to cache the dcs as well and revalidate them

Open question:
Is there any risk for a server accepting a ticket associated with a DC to resume a connection if it would not have issued a DC on that connection for a full handshake. I think the answer to this is no risk.