Revert "Optimizations for ReadOnlyEngine"

This reverts commit e57c2d9.

Author: kingherc

server/src/main/java/org/elasticsearch/index/engine/ReadOnlyEngine.java

@@ -358,7 +358,9 @@ public Closeable acquireHistoryRetentionLock() {
 
     @Override
     public int countChanges(String source, long fromSeqNo, long toSeqNo) throws IOException {
-        return 0;
+        try (Translog.Snapshot snapshot = newChangesSnapshot(source, fromSeqNo, toSeqNo, false, true, true, -1)) {
+            return snapshot.totalOperations();
+        }
     }
 
     @Override
@@ -500,9 +502,7 @@ public SafeCommitInfo getSafeCommitInfo() {
     }
 
     @Override
-    public void activateThrottling() {
-        assert false : "read only engine does not ingest and thus should not be throttled";
-    }
+    public void activateThrottling() {}
 
     @Override
     public void deactivateThrottling() {}