backup: switch over to streaming Iterator improving memory usage

Avoid collecting the whole group list in memory only to iterate and
filter over it again.

Note that the change could result in a indentation change, so best
viewed with `-w` flag.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

src/server/pull.rs

@@ -797,7 +797,8 @@ pub async fn pull_store(
 
     if params.remove_vanished {
         let result: Result<(), Error> = proxmox_lang::try_block!({
-            for local_group in params.store.list_backup_groups()? {
+            for local_group in params.store.iter_backup_groups()? {
+                let local_group = local_group?;
                 if new_groups.contains(&local_group) {
                     continue;
                 }