No string in slog (#2017)

## Issue Addressed

Following slog's documentation, this should help a bit with string allocations. I left it run for two days and mem usage is lower. This is of course anecdotal, but shouldn't harm anyway 

## Proposed Changes

remove `String` creation in logs when possible

beacon_node/network/src/beacon_processor/mod.rs

@@ -961,7 +961,7 @@ impl Drop for SendOnDrop {
                 self.log,
                 "Unable to free worker";
                 "msg" => "did not free worker, shutdown may be underway",
-                "error" => e.to_string()
+                "error" => %e
             )
         }
     }