Support lazy open union children

[lysu]

Development Task

In #19827, we support concurrency control for Union's children execution.

but Union executor still Init all children before execute Next

init children will cause sub-plan nodes's Init be called

combined with #14320, partition table will make the "buildCopTasks takes too much time" problem even worse.

so for a partition table with 128 partitions

select * from partition_table limit 1;

it will build all copTask in memory for each 128 partitions before send any request to tikv(and finally maybe only 1 request is useful)

[tiancaiamao]

There is such an attempt some time ago #16815

[XuHuaiyu]

reopen this issue cause the fix is reverted in #23276