Warn for runid changing in load events; add restore_load_context flag

Warn for runid changing in load events; add restore_load_context flag

Added a new flag :paramref:`.InstanceEvents.restore_load_context` and
:paramref:`.SessionEvents.restore_load_context` which apply to the
:meth:`.InstanceEvents.load`, :meth:`.InstanceEvents.refresh`, and
:meth:`.SessionEvents.loaded_as_persistent` events, which when set will
restore the "load context" of the object after the event hook has been
called.  This ensures that the object remains within the "loader context"
of the load operation that is already ongoing, rather than the object being
transferred to a new load context due to refresh operations which may have
occurred in the event. A warning is now emitted when this condition occurs,
which recommends use of the flag to resolve this case.  The flag is
"opt-in" so that there is no risk introduced to existing applications.

The change additionally adds support for the ``raw=True`` flag to
session lifecycle events.

Fixes: #5129
Change-Id: I2912f48ac8c5636297d63ed383454930e8e9a6a3