Always use NORMAL ReadAdvice with cfs files (elastic#135831)

This works around an issue where RANDOM advice was being used for flushes, which causes a significant increase in IOPS and a drop in performance.

Author: thecoop

server/src/main/java/org/elasticsearch/index/store/FsDirectoryFactory.java

@@ -157,6 +157,9 @@ private static BiFunction<String, IOContext, Optional<ReadAdvice>> getReadAdvice
             if (context.hints().contains(StandardIOBehaviorHint.INSTANCE)) {
                 return Optional.of(ReadAdvice.NORMAL);
             }
+            if (name.endsWith(".cfs")) {
+                return Optional.of(ReadAdvice.NORMAL);
+            }
             return MMapDirectory.ADVISE_BY_CONTEXT.apply(name, context);
         };
     }