Add TODO comment to warn future delete case

src/neo/Ledger/Blockchain.cs

@@ -489,6 +489,7 @@ private void Persist(Block block)
                 }
                 snapshot.Blocks.Add(block.Hash, block.Trim());
                 StoreView clonedSnapshot = snapshot.Clone();
+                // TODO: Handle occassions where Datacache.Delete() is called by variable snapshot
                 foreach (Transaction tx in block.Transactions)
                 {
                     var state = new TransactionState