reduce comment cache size

it may mitigate safari indexedDB data usage bug, but i'm not sure

src/cache.js

@@ -1,7 +1,7 @@
 const HOLE_CACHE_DB_NAME='hole_cache_db';
 const CACHE_DB_VER=1;
-const MAINTENANCE_STEP=200;
-const MAINTENANCE_COUNT=2500;
+const MAINTENANCE_STEP=150;
+const MAINTENANCE_COUNT=1000;
 
 const ENC_KEY=42;