Allow multiple parent certificates

[cyrill-k]

Currently, each certificate is tied to a single parent certificate.
However, this is not always the case, e.g., if a root or intermediate certificate is renewed with the same subject name and public key.
This could be solved by adding the "issuer identity", which is the tuple of (subject name, subject key identifier, signature algorithm), as an additional field to the parent/child table in the database.

[juagargi]

Note for self:

Be careful during ingestion time, that the certificates that are ingested are not used as issuers by any existing certificates.
It they are, we recursively find any certificate that is a descendant of any cert in the set. Then finding the unique set of domains that have any of these certificates using the domain_certs table (which will have to have an index on cert_id).