resgroup: disable runaway detection when resource group is enabled
resgroup: disable runaway detection when resource group is enabled

Runaway system pick one session who consumed the most memories and
notify it to clean up once red-zone is hit, meanwhile, memories are
isolated between groups in resource group which means one query in
a resource group may cancel queries within another resource group.

To avoid this, we do not do runaway detection when resource group
is enabled, this means GUC runaway_detector_activation_percent takes
no effect under resource group.

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