Implement SQLite ON CONFLICT for constraints

Implement SQLite ON CONFLICT for constraints

Implemented the SQLite ``ON CONFLICT`` clause as understood at the DDL
level, e.g. for primary key, unique, and CHECK constraints as well as
specified on a :class:`.Column` to satisfy inline primary key and NOT NULL.
Pull request courtesy Denis Kataev.

Fixes: #4360
Change-Id: I4cd4bafa8fca41e3101c87dbbfe169741bbda3f4
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/431