Moved leveldb update loop to eth/backend

change order of block insert and update LastBlock

bugfix, wrong hash stored in blockDb

ethdb/memory_database.go

@@ -65,3 +65,7 @@ func (db *MemDatabase) LastKnownTD() []byte {
 
 	return data
 }
+
+func (db *MemDatabase) Flush() error {
+	return nil
+}