Implement relationship AND criteria; global loader criteria

Implement relationship AND criteria; global loader criteria

Added the ability to add arbitrary criteria to the ON clause generated
by a relationship attribute in a query, which applies to methods such
as :meth:`_query.Query.join` as well as loader options like
:func:`_orm.joinedload`.   Additionally, a "global" version of the option
allows limiting criteria to be applied to particular entities in
a query globally.

Documentation is minimal at this point, new examples will
be coming in a subsequent commit.

Some adjustments to execution options in how they are represented
in the ORMExecuteState as well as well as a few ORM tests that
forgot to get merged in a preceding commit.

Fixes: #4472
Change-Id: I2b8fc57092dedf35ebd16f6343ad0f0d7d332beb