Summary:
WritePreparedTransactionDB operates with more options which should not be configurable to avoid complicating it for the users. For testing purposes however we need to change the default value of this parameters. This patch makes these parameters private fields in TransactionDBOptions so that the existing ::Open API could use them seamlessly without however exposing them to the users.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4966

Differential Revision: D14015986

Pulled By: maysamyabandeh

fbshipit-source-id: 13037efa7dfdd6f73ec7a19414b66571e044c633