A transaction that conflicts against itself, for example
	INSERT INTO t(pk) VALUES (1),(1) ON CONFLICT DO NOTHING;
should behave the same regardless of isolation level.  It certainly
shouldn't throw a serialization error, as retrying will not help.
We got this wrong due to the ON CONFLICT logic not considering the case,
as reported by Jason Dusek.

Core of this patch is by Peter Geoghegan (based on an earlier patch by
Thomas Munro), though I didn't take his proposed code refactoring for fear
that it might have unexpected side-effects.  Test cases by Thomas Munro
and myself.

Report: <CAO3NbwOycQjt2Oqy2VW-eLTq2M5uGMyHnGm=RNga4mjqcYD7gQ@mail.gmail.com>
Related-Discussion: <57EE93C8.8080504@postgrespro.ru>