Prune process allows for orphaned certs

[juagargi]

Because it is possible that an issuer certificate expires before a child one (mathematically possible, at least), the prune process could leave some certs entries pointing to their issuer by using an issuer_id (aka parent_id) that doesn't exist in the table.

Modify the ingestion process so that it doesn't break, in particular the domain payload ID coalescing stored procedures.
Add a test to ensure this is correct.

[juagargi]

PR #44 made a modification on how the pruning works, check if this issue is still present.