Unify loading of a snapshot and loading the initial DB on disk.

[Leo-Besancon]

Right now, before genesis (and if keep-ledger is not set), we load the initial ledger by manually adding the key values passed via config files.

We should instead generate an "Initial DB" and load it in the same way we would a snapshot, unifying two different behaviours.

Originally addressed by @damip in https://github.com/massalabs/massa/pull/4197#discussion_r1257212999