Startup optimization in shrink - don't shrink non-shrinkable slots (backport #17405) (#17792)

* Skip shrink when it doesn't save anything (#17405)

(cherry picked from commit 14c52ab018)

# Conflicts:
#	runtime/src/accounts_db.rs

* fix merge error

Co-authored-by: sakridge <sakridge@gmail.com>
Co-authored-by: Jeff Washington (jwash) <wash678@gmail.com>

runtime/src/accounts_index.rs

@@ -146,6 +146,10 @@ impl<T: Clone> ReadAccountMapEntry<T> {
     pub fn unref(&self) {
         self.ref_count().fetch_sub(1, Ordering::Relaxed);
     }
+
+    pub fn addref(&self) {
+        self.ref_count().fetch_add(1, Ordering::Relaxed);
+    }
 }
 
 #[self_referencing]