If a LIMIT is applied to a UNION ALL query, plan each UNION arm as

If a LIMIT is applied to a UNION ALL query, plan each UNION arm as
if the limit were directly applied to it.  This does not actually
add a LIMIT plan node to the generated subqueries --- that would be
useless overhead --- but it does cause the planner to prefer fast-
start plans when the limit is small.  After an idea from Phil Endecott.