Move saving features to `system.local#supported_features`
to the point after passing all remote feature checks in
the gossiper, right before joining the ring.
This makes `system.local#supported_features` column to store
advertised feature set. Leave a comment in the definition of
`system.local` schema to reflect that.
Since the column value is not actually used anywhere for now,
it shouldn't affect any tests or alter the existing behavior.
Later, we can optimize the gossip communication between nodes
in the cluster, removing the feature check altogether
in some cases (since the column value should now be monotonic).
* manmanson/save_adv_features_v2:
db: save supported features after passing gossip feature check
db: add `save_local_supported_features` function
2a3450dfb7