Summary:
Add oldest snapshot sequence property, so we can use `db.GetProperty("rocksdb.oldest-snapshot-sequence")` to get the sequence number of the oldest snapshot.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6228

Differential Revision: D19264145

Pulled By: maysamyabandeh

fbshipit-source-id: 67fbe5304d89cbc475bd404e30d1299f7b11c010