expand begin_transaction sync config settings

Dmitri has asked that instead of a boolean setting, a user be allowed to set a specific setting on a transaction to get not just none/fsync but write-no-sync or in-memory buffering or other settings. I.e. someone running fsync may want to relax the setting to write-no-sync. Someone with in-memory may want to enforce write-no-sync, but not necessarily fsync, etc.