Extract format_constraint truncation rules to ON CONFLICT

Extract format_constraint truncation rules to ON CONFLICT

Fixed issue where a too-long constraint name rendered as part of the "ON
CONFLICT ON CONSTRAINT" element of the :class:`_postgresql.Insert`
construct due to naming convention generation would not correctly truncate
the name in the same way that it normally renders within a CREATE TABLE
statement, thus producing a non-matching and too-long constraint name.

Fixes: #6755
Change-Id: Ib27014a5ecbc9cd5861a396f8bb49fbc60bf49fe