Summary:
The assertion was caught by `MySQLStyleTransactionTest/MySQLStyleTransactionTest.TransactionStressTest/5` when run in a loop. The caller doesn't track whether the released snapshot is oldest, so let this function handle that case.
Closes https://github.com/facebook/rocksdb/pull/3080

Differential Revision: D6185257

Pulled By: ajkr

fbshipit-source-id: 4b3015c11db5d31e46521a00af568546ef4558cd