Explicit db.flush() does not work in OptimisticTransactionDB

[pragmaxim]

Repro https://github.com/pragmaxim-com/rust-playground/commit/e5ec1c6956d8868a6e09257fc71fb40fada2bab4

db.flush call does not lead to new (the first) sst file being created ... not even after thread-sleep 10 seconds after commit or after flushing