Postgres Backend for CFSSL

It would be useful to have a permanent store for CFSSL/multirootca in the form of a postgres database. This database can also be used as a source of truth about the status of the issued certificates, including revocation status.

• Create a database schema for certificates
• Add issued certificates to the database

Some additional tools should be created for interacting with this database. Specifically:

• revoke: a tool that takes a certificate or serial number, and the revocation reason and marks the certificate in the database as revoked
• oscp: a tool that queries all unexpired certificates from the database and creates an OCSP response for each using the CFSSL OCSPSign API. The output is a file containing concatenated OCSP responses in a format that cfssl oscpserve can use.
• crl: a tool that queries all unexpired certificates from the database and creates a CRL using CFSSL's crl endpoint.