Be clear about what CertCreation is

From !20 (comment 43023)

  "the last createdOn value" # helper field to avoid looking for all CertCreation
  "the first block number of the certification creation" # helper field to avoid looking for all CertCreation

It is not clear at the moment what "cert created on" is:

• the first creation of the cert?
• the last one (ex: created - expired - re-created)

It could be useful to have both fields.