feat: Enforce 1 user 1 connection for X-ray and fix active stats speed

migrations/052_add_current_speed_to_clients.sql

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+ALTER TABLE vpn_clients ADD COLUMN current_speed BIGINT DEFAULT 0 AFTER traffic_limit;

migrations/053_split_speed.sql

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+ALTER TABLE vpn_clients ADD COLUMN speed_up BIGINT DEFAULT 0 AFTER current_speed;
+ALTER TABLE vpn_clients ADD COLUMN speed_down BIGINT DEFAULT 0 AFTER speed_up;
+-- We can drop current_speed later or keep it as total