The `system.topology` table is extended with two new columns that will
be used to manage cluster features:
- `supported_features set<text>` is a new clustering column which holds
the features that given node advertises as supported. It will be first
initialized when the node joins the cluster, and then updated every
time the node reboots and its supported features set changes.
- `enabled_features set<text>` is a new static column which holds the
features that are considered enabled by the cluster. Unlike in the
current gossip-based implementation the features will not be enabled
implicitly when all nodes support a feature, but rather when via an
explicit action of the topology coordinator.
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