No need for files_by_size_ in universal compaction

Summary: files_by_size_ is sorted by time in case of universal compaction. However, Version::files_ is also sorted by time. So no need for files_by_size_

Test Plan:
1) make check with the change
2) make check with `assert(last_index == c->input_version_->files_[level].size() - 1);` in compaction picker

Reviewers: dhruba, haobo, yhchiang, sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19125
Fix compile error
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