Handle interrupts while waiting on Append's async subplans

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: http://www.postgresql.org/message-id/37a40570-f558-40d3-b5ea-5c2079b3b30b@iki.fi