Summary:
1. fix segment error when dumping old sst format (no properties nor stats)
2. Enable dumpping old sst format
Test Plan:
Generate block based sst file with "properties", and one with "stats" and one without neither.
Read it using sst_dump
Reviewers: ljin, igor, yhchiang, dhruba, sdong
Reviewed By: sdong
Subscribers: leveldb
Differential Revision: https://reviews.facebook.net/D21837
6c4c159b2c