Fix benchmarks: set cache_size to higher value (#5585)

* 128 -> 1024

* add trace and set = 512

* fix doc comment

substrate/bin/node/testing/src/bench.rs

@@ -186,7 +186,7 @@ impl BenchDb {
 			pruning: PruningMode::ArchiveAll,
 			source: sc_client_db::DatabaseSettingsSrc::Path {
 				path: dir.into(),
-				cache_size: 128,
+				cache_size: 512,
 			},
 		};