fix tracking oldest snapshot for bottom-level compaction

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

db/db_impl.cc

@@ -1618,7 +1618,9 @@ void DBImpl::ReleaseSnapshot(const Snapshot* s) {
     snapshots_.Delete(casted_s);
     uint64_t oldest_snapshot;
     if (snapshots_.empty()) {
-      oldest_snapshot = versions_->LastSequence();
+      oldest_snapshot = concurrent_prepare_ && seq_per_batch_
+                              ? versions_->LastToBeWrittenSequence()
+                              : versions_->LastSequence();
     } else {
       oldest_snapshot = snapshots_.oldest()->number_;
     }