Handle interrupts while waiting on Append's async subplans
We did not wake up on interrupts while waiting on async events on an
async-capable append node. For example, if you tried to cancel the
query, nothing would happen until one of the async subplans becomes
readable. To fix, add WL_LATCH_SET to the WaitEventSet.
Backpatch down to v14 where async Append execution was introduced.
Discussion: https://www.postgresql.org/message-id/
37a40570-f558-40d3-b5ea-
5c2079b3b30b@iki.fi