This supports the case where we have a non-exclusive grant that has rx-only ports,
and that does not conflict with other transmitters at schedule time (yet; possible
in future once we have IntConstraints).  May conflict at runtime; alarm service
will tell us.

Includes a variety of bugfixes for various conflict cases, and sneaks in some
additional db fields we need, e.g. to model the pending state of grants that are
blocked due to a conflict.