Always use a separate database for blobs (#4892)

* Always use a separate blobs DB

* Add + update tests

beacon_node/beacon_chain/src/data_availability_checker/overflow_lru_cache.rs

@@ -771,12 +771,13 @@ mod test {
     ) -> Arc<HotColdDB<E, LevelDB<E>, LevelDB<E>>> {
         let hot_path = db_path.path().join("hot_db");
         let cold_path = db_path.path().join("cold_db");
+        let blobs_path = db_path.path().join("blobs_db");
         let config = StoreConfig::default();
 
         HotColdDB::open(
             &hot_path,
             &cold_path,
-            None,
+            &blobs_path,
             |_, _, _| Ok(()),
             config,
             spec,