features: add feature flag for removenode via left token ring

To improve the behavior of the removenode operation, we want to issue
a global topology barrier after the removenode has been applied.
However, this requires changing the topology state machine to add a new
state (left_token_ring) to the removenode flow, which is not supported
by older nodes.

To allow rolling upgrades, we add a feature flag
REMOVENODE_WITH_LEFT_TOKEN_RING that controls whether the new removenode
flow is used.

gms/feature_service.hh

@@ -177,6 +177,7 @@ public:
     gms::feature driver_service_level { *this, "DRIVER_SERVICE_LEVEL"sv };
     gms::feature strongly_consistent_tables { *this, "STRONGLY_CONSISTENT_TABLES"sv };
     gms::feature client_routes { *this, "CLIENT_ROUTES"sv };
+    gms::feature removenode_with_left_token_ring { *this, "REMOVENODE_WITH_LEFT_TOKEN_RING"sv };
 public:
 
     const std::unordered_map<sstring, std::reference_wrapper<feature>>& registered_features() const;