liveslots relaxDurabilityMode admits Promises when it should reject them

[warner]

We have "fake vat" testing environments which configure their copy of
liveslots with a setting named relaxDurabilityRules = true. This
changes the rules for durable objects, which normally refuse to accept
non-durable state. When the rules are relaxed, they accept ephemeral
and merely-virtual objects as well.

Promises were never supposed to be accepted, even when the rules are
relaxed. But liveslots has a bug, and accepts Promises in the relaxed
mode.

Lacking this enforcement, some other packages had mistakenly stored
Promises in durable objects, and gotten away with it. When we fix liveslots to not accept Promises in durable state, we will also need to update those packages to follow the rules.