chore: minor refactorings around dense_set deletions (#4390)

chore: refactorings around deletions

Done as a preparation to introduce asynchronous deletions for sets/zsets/hmaps.
1. Restrict the interface around DbSlice::Del. Now it requires for the iterator to be valid and the checks should
be explicit before the call. Most callers already provides a valid iterator.

2. Some minor refactoring in compact_object_test.
3. Expose DenseSet::ClearStep to allow iterative deletions of elements.

Signed-off-by: Roman Gershman <roman@dragonflydb.io>

src/server/db_slice.h

@@ -346,7 +346,8 @@ class DbSlice {
   // Delete a key referred by its iterator.
   void PerformDeletion(Iterator del_it, DbTable* table);
 
-  bool Del(Context cntx, Iterator it);
+  // Deletes the iterator. The iterator must be valid.
+  void Del(Context cntx, Iterator it);
 
   constexpr static DbIndex kDbAll = 0xFFFF;