rename cached_traffic_control.rs to traffic_control_cache.rs, improve dev docs

Keep things inside crate::traffic_control_cache (do not pollute root namespace).

Signed-off-by: Dietmar Maurer <dietmar@proxmox.com>

src/api2/admin/traffic_control.rs

@@ -8,7 +8,7 @@ use pbs_api_types::{
     TrafficControlRule, PRIV_SYS_AUDIT,
 };
 
-use crate::TRAFFIC_CONTROL_CACHE;
+use crate::traffic_control_cache::TRAFFIC_CONTROL_CACHE;
 
 #[api(
     properties: {