Information about the number of shares per service level will be stored
in an additional column in the service levels table, which is managed
through group0. We will need the feature to make sure that all nodes in
the cluster know about the new column before any node starts applying
group0 commands the would touch the new column.
This feature also serves a role for the legacy service levels
implementation that uses system_distributed for storage: after all nodes
are upgraded to support workload prioritization, one of the nodes will
perform a schema change operation and will add the new column.
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