Optimize out JOIN for selectinload with many to one

Optimize out JOIN for selectinload with many to one

The optimzation applied to selectin loading in :ticket:`4340` where a JOIN
is not needed to eagerly load related items is now applied to many-to-one
relationships as well, so that only the related table is queried for a
simple join condition.   In this case, the related items are queried
based on the value of a foreign key column on the parent; if these columns
are deferred or otherwise not loaded on any of the parent objects in
the collection, the loader falls back to the JOIN method.

Fixes: #4775
Change-Id: I398e0d276789ce6e0019d213a37bdf99d24ec290