LevelDB_TTL_/TODO

Before adding to chrome
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- multi-threaded test/benchmark
- Allow missing crc32c in Table format?

Maybe afterwards
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ss
- Stats
- Speed up backwards scan (avoid three passes over data)

db
- Maybe implement DB::BulkDeleteForRange(start_key, end_key)
  that would blow away files whose ranges are entirely contained
  within [start_key..end_key]?  For Chrome, deletion of obsolete
  object stores, etc. can be done in the background anyway, so
  probably not that important.

api changes?
- Efficient large value reading and writing

Faster Get implementation