disktable and memtable behave inconsistently with max_traverse_cnt

When traversing a table, we use `max_traverse_cnt` to control. If `iterator->GetCount() >= max_traverse_cnt`, the traverse will be stopped. This decision is mainly in tablet_impl.cc but can be done early in disk_table.cc and mem_table.cc. For now, we add more `if (iterator->GetCount() >= max_traverse_cnt)` in disktable, which makes the `max_traverse_cnt` flag works in disktable but not in memtable. In consequence, some tests in table_test related to `max_traverse_cnt` have different results for memtable and disktable