Issue facebook#33: SELECT ... FROM rocksdb_table ORDER BY primary_key…

… uses sorting

Summary:
Fix the index properties returned by the storage engine:
- ha_rocksdb::primary_key_is_clustered()= true;
- ha_rocksdb::keys_to_use_for_scanning()= &key_map_full;

Differential Revision: https://reviews.facebook.net/D33957

storage/rocksdb/ha_rocksdb.h

@@ -289,6 +289,15 @@ class ha_rocksdb: public handler
   */
   ulong index_flags(uint inx, uint part, bool all_parts) const;
 
+  const key_map * keys_to_use_for_scanning()
+  {
+    return &key_map_full;
+  }
+  bool primary_key_is_clustered()
+  {
+    return true;
+  }
+
   int rename_table(const char *from, const char *to);
 
   int convert_record_from_storage_format(rocksdb::Slice *slice, uchar *buf);