Speed up reduce_levels_test

Summary: For some reason reduce_levels is opening the databse with 65.000 levels. This makes ComputeCompactionScore() function terribly slow and the tests is also very slow (20seconds).

Test Plan: mr reduce_levels_test now takes 20ms

Reviewers: sdong, rven, kradhakrishnan, yhchiang

Reviewed By: yhchiang

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D37059

util/ldb_cmd.cc

@@ -1084,7 +1084,7 @@ ReduceDBLevelsCommand::ReduceDBLevelsCommand(const vector<string>& params,
       const map<string, string>& options, const vector<string>& flags) :
     LDBCommand(options, flags, false,
                BuildCmdLineOptions({ARG_NEW_LEVELS, ARG_PRINT_OLD_LEVELS})),
-    old_levels_(1 << 16),
+    old_levels_(1 << 7),
     new_levels_(-1),
     print_old_levels_(false) {