Simplify the {Cert,Vote}DB interface, removing the `Snapshot` intermediary type

[tbagrel1]

At the moment the Peras{Cert,Vote}DB expose a {Cert,Vote}Snapshot. Initially this has been done in order to separe the STM part from the monadic/IO part, but we didn't really follow up on this in a consistent way (see IntersectMBO/ouroboros-consensus#1768 (comment) for more details).

We should probably remove or at least simplify this interface.