skip packing multi ref ancient slots until sufficient normal slots ar…

…e packed (#1583)

* don't pack multi ref slots until safe

* sub 1 to get math perfect

accounts-db/src/accounts_db.rs

@@ -1942,6 +1942,7 @@ pub(crate) struct ShrinkAncientStats {
     pub(crate) slots_considered: AtomicU64,
     pub(crate) ancient_scanned: AtomicU64,
     pub(crate) bytes_ancient_created: AtomicU64,
+    pub(crate) many_ref_slots_skipped: AtomicU64,
 }
 
 #[derive(Debug, Default)]
@@ -2231,6 +2232,11 @@ impl ShrinkAncientStats {
                 self.bytes_ancient_created.swap(0, Ordering::Relaxed) as i64,
                 i64
             ),
+            (
+                "many_ref_slots_skipped",
+                self.many_ref_slots_skipped.swap(0, Ordering::Relaxed),
+                i64
+            ),
         );
     }
 }