Allow SQL expression for ORM primary keys

Allow SQL expression for ORM primary keys

A SQL expression can now be assigned to a primary key attribute for an ORM
flush in the same manner as ordinary attributes as described in
:ref:`flush_embedded_sql_expressions` where the expression will be evaulated
and then returned to the ORM using RETURNING, or in the case of pysqlite,
works using the cursor.lastrowid attribute.Requires either a database that
supports RETURNING (e.g. Postgresql, Oracle, SQL Server) or pysqlite.

Fixes: #3133
Fixes: #4494
Change-Id: I83da8357354de002cb04fa4a553f2a2f90c5157d