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

Look at tablet_impl.cc, disk_table.cc, and mem_table.cc for the traversal logic and the check on iterator->GetCount() >= max_traverse_cnt. The inconsistency is that disk_table has this check but mem_table does not. Run the table_test tests related to max_traverse_cnt to see the differing results. The fix is to ensure the same early stop logic is applied in both table implementations.

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