implement join_depth for immediateload, fix expunged loading

implement join_depth for immediateload, fix expunged loading

Fixed issue where the ``lazy="immediateload"`` loader strategy would place
an internal loading token into the ORM mapped attribute under circumstances
where the load should not occur, such as in a recursive self-referential
load.   As part of this change, the ``lazy="immediateload"`` strategy now
honors the :paramref:`_orm.relationship.join_depth` parameter for
self-referential eager loads in the same way as that of other eager
loaders, where leaving it unset or set at zero will lead to a
self-referential immediateload not occurring, setting it to a value of one
or greater will immediateload up until that given depth.

Fixes: #10139
Change-Id: I319d8cd86d7dd57f8ba61a9d483dc84505e02c72