In tools/db_stress.cc, set proper value in NewHashSkipListRepFactory's bucket_size

Summary:
    Now that the arena is used to allocate space for hashskiplist's bucket. The bucket size
    need to be set small enough to avoid "should_flush_" failure in memtable's assertion.

Test Plan:
    make all check

Reviewers: sdong

Reviewed By: sdong

Subscribers: igor

Differential Revision: https://reviews.facebook.net/D19371

tools/db_stress.cc

@@ -1591,7 +1591,7 @@ class StressTest {
     }
     switch (FLAGS_rep_factory) {
       case kHashSkipList:
-        options_.memtable_factory.reset(NewHashSkipListRepFactory());
+        options_.memtable_factory.reset(NewHashSkipListRepFactory(10000));
         break;
       case kSkipList:
         // no need to do anything