fix #3111: config: snapshot delete: check if replication still needs it

fix #3111: config: snapshot delete: check if replication still needs it

and abort if it does and --force is not specified.

After rollback, the rollback snapshot might still be needed as the
base for incremental replication, because rollback removes (blocking)
replication snapshots.

It's not enough to limit the check to the most recent snapshot,
because new snapshots might've been created between rollback and
remove.

It's not enough to limit the check to snapshots without a parent (i.e.
in case of ZFS, the oldest), because some volumes might've been added
only after that, meaning the oldest snapshot is not an incremental
replication base for them.

Signed-off-by: Fabian Ebner <f.ebner@proxmox.com>