[Issue 9011] add mdb_txn_flags()

https://bugs.openldap.org/show_bug.cgi?id=9011

--- Comment #3 from christopher@gmerlin.de christopher@gmerlin.de --- I don't remember the exact reason why I needed this function. But I can offer general remarks. OCaml is a strictly statically typed language. The lmdb binding attaches phantom types to the lmdb data types: a rw/ro type variable to transactions and a dup/nodup type variable to databases. This allows catching some errors like writes with a read-only transaction at compile time. A possible use of mdb_txn_flags() could be to check an _runtime_ that those invariants are not broken when a raw transaction handle (without phantom types) is imported into the typed binding.