Rebase Postgres snapshot modes on exported snapshots

As revealed by DBZ-2288, those snapshotting modes which are not using a snapshot exported by the replication slot are prone to missing events originating from concurrent transactions that happen while the snapshot is running.

There really is no reason to not using exported snapshots, so we should always leverage them. This means:

• Make use of exported snapshots for all snapshot modes
• Deprecate the "exported" mode; for now, it will simply be an alias to "initial"