Werk #5459: Disallow group-related wait objects, avoiding crashes

Using wait objects on some tables (hostsbygroup, servicesbygroup, servicesbyhostgroup) doesn't really make sense: Hosts and services can be part of many groups, so we would have to conjure up a random group out of thin air. Consequently, we forbid wait objects on these tables now. Previously, the monitoring core crashed when we would try to wait on such an object, anyway.

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