Tweak notin test.
Tweak RSE test case.
Fix bug in RSE when negation contains qual list.

When applying RSE to a negation whose left input is a join, we didn't account
for the possibility that the negation might have a set of quals as well.
Optimize RCE to only send key values in ACKs.

This is sufficient, because the keys (at the sender) functionally determine the
rest of the outbound message. By only sending keys, we reduce the size of the
ACKs; moreover, this means the sender-side negation need consider fewer columns,
which will allow us to implement it more efficiently in the future.