core: allow to perform storage search within given amount of DAO layers

pkg/core/storage/memcached_store.go

@@ -301,7 +301,12 @@ func performSeek(ctx context.Context, ps Store, memRes []KeyValueExists, rng See
 			}
 		}
 	}
-	ps.Seek(rng, mergeFunc)
+	if rng.SearchDepth == 0 || rng.SearchDepth > 1 {
+		if rng.SearchDepth > 1 {
+			rng.SearchDepth--
+		}
+		ps.Seek(rng, mergeFunc)
+	}
 
 	if !done && haveMem {
 	loop:

pkg/core/storage/store.go

@@ -59,6 +59,11 @@ type SeekRange struct {
 	// whether seeking should be performed in a descending way.
 	// Backwards can be safely combined with Prefix and Start.
 	Backwards bool
+	// SearchDepth is the depth of Seek operation, denotes the number of cached
+	// DAO layers to perform search. Use 1 to fetch the latest changes from upper
+	// in-memory layer of cached DAO. Default 0 value denotes searching through
+	// the whole set of cached layers.
+	SearchDepth int
 }
 
 // ErrKeyNotFound is an error returned by Store implementations