Fix ON CONFLICT DO NOTHING/UPDATE for temporal indexes

Fix ON CONFLICT DO NOTHING/UPDATE for temporal indexes

A PRIMARY KEY or UNIQUE constraint with WITHOUT OVERLAPS will be a
GiST index, not a B-Tree, but it will still have indisunique set.  The
code for ON CONFLICT fails if it sees a non-btree index that has
indisunique.  This commit fixes that and adds some tests.  But now
that we can't just test indisunique, we also need some extra checks to
prevent DO UPDATE from running against a WITHOUT OVERLAPS constraint
(because the conflict could happen against more than one row, and we'd
only update one).

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/1426589a-83cb-4a89-bf40-713970c07e63@illuminatedcomputing.com