WL#3688: Implement new syntax for UPDATE's where also the READ BEFORE can be used

The idea is to allow handlers in one call to
both read and update a primary key or unique
key row using a list of fields to read and
updating using simple expressions.

In this case one can avoid going to the handler
multiple times for some operations often found
in OLTP applications.

E.g.
UPDATE t1 SET a = a + 1
WHERE PK = 1
RETURNING a, b, c, d INTO out_a, out_b, out_c, out_d;

This is equivalent to the statements:
SELECT a,b,c,d into out_a, out_b, out_c, out_d from t1 where PK = 1 FOR UPDATE;
UPDATE t1 SET a = out_a + 1 WHERE PK = 1;