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/bin/proxmox-backup-proxy.rs

@@ -40,7 +40,7 @@ use proxmox_backup::{
         auth::check_pbs_auth,
         jobstate::{self, Job},
     },
-    TRAFFIC_CONTROL_CACHE,
+    traffic_control_cache::TRAFFIC_CONTROL_CACHE,
 };
 
 use pbs_buildcfg::configdir;