iter: Add iteration mode where session is not busy

iter: Add iteration mode where session is not busy

In order to use the session we are iterating on for other tasks
such as other C_FindObject() calls, we need to make sure that
it's not in the middle of a find operation. Finish up the
complete find operation in advance of returning objects from
a session.

Make this the default mode. The previous behavior remains
as an option. Add tests.