db_bench --num_hot_column_families to be default off

Summary: Having --num_hot_column_families default on fails some existing regression tests. By default turn it off

Test Plan: Run db_bench to make sure it is default off.

Reviewers: yhchiang, rven, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D30705

db/db_bench.cc

@@ -154,7 +154,7 @@ DEFINE_int64(merge_keys, -1,
 DEFINE_int32(num_column_families, 1, "Number of Column Families to use.");
 
 DEFINE_int32(
-    num_hot_column_families, 8,
+    num_hot_column_families, 0,
     "Number of Hot Column Families. If more than 0, only write to this "
     "number of column families. After finishing all the writes to them, "
     "create new set of column families and insert to them. Only used "