With this change the certificate chain sent by the server will actually
be verified, instead of just checking the names of the certificates.
To determine if a certificate is signed by a root certificate, the list
of root certificates is now a HashMap mapping from the unique identifier
string to the certificate. This allows us to take the issuer of a
certificate and easily check if it is a root certificate. If a
certificate is not signed by a root certificate, we will check that it
is signed by the next certificate in the chain.
This also removes the ad-hoc checking of certificate validity from
multiple places, and moves all checking to the verify_chain.
