Refactor logging levels to debug for improved performance and clarity

- Changed several logging statements from `info` to `debug` across various modules, including `StdBackend`, `RefPathIndex`, `ServeSession`, and `syncback_loop_with_stats`.
- This adjustment aims to reduce log verbosity during normal operations while retaining detailed logs for debugging purposes, enhancing overall performance and maintainability.

src/rojo_ref.rs

@@ -428,7 +428,7 @@ impl RefPathIndex {
         }
 
         if count > 0 {
-            log::info!(
+            log::debug!(
                 "RefPathIndex: populated {} Rojo_Ref_* entries from existing meta/model files",
                 count
             );