Rework to use real database

[wneirynck]

Using bucket files as database is showing its limitations. Not only is it slow, but it also limits our possibilities to retrieve data. Currently this is fine for most situations, but we already receive timeouts for repos with many builds. So we'd better replace this with a "real" database, like MySQL.

MySQL also supports JSON fields, so we could store the more dynamic parts of the entities (like job configuration and results) in JSON. The biggest drawback is that we become dependent on another external service, although it may be better than the current solution (file storage).

[wneirynck]

Later we could move to a managed MySQL cluster, provided by OCI.