Fixes for lambda expressions and relationship loaders

Fixes for lambda expressions and relationship loaders

Fixed bug in lambda SQL feature, used by ORM
:meth:`_orm.with_loader_criteria` as well as available generally in the SQL
expression language, where assigning a boolean value True/False to a
variable would cause the query-time expression calculation to fail, as it
would produce a SQL expression not compatible with a bound value.

Fixed issue where the :attr:`_orm.ORMExecuteState.is_relationship_load`
parameter would not be set correctly for many lazy loads, all
selectinloads, etc.  The flag is essential in order to test if options
should be added to statements or if they would already have been propagated
via relationship loads.

Fixes: #5763
Fixes: #5764
Change-Id: I66aafbef193f892ff75ede0670698647b7475482