[data] estimate blocks for limit and union operator

[Zandew]

Why are these changes needed?

We estimate # of blocks outputted for MapOperator, but it depends on the num_outputs_total of it's input dependency. If this method is allowed to return None, then the output blocks of any operator following it cannot be estimated.

This pr estimates the number of blocks outputted from a LimitOperator using the rule: _consumed_rows / _limit = _cur_output_bundles / *total_output_bundles*. This and UnionOperator were the only other operators that returned None, so now num_outputs_total will always return an int.

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