Default to DB_TXN_WRITE_NOSYNC for all transactional operations

Default to DB_TXN_WRITE_NOSYNC for all transactional operations

* This is safer than DB_TXN_NOSYNC, and does not appear to impact
  performance.
* Applying this to the dbenv is necessary to avoid many fdatasync(2)
  calls on db 5.x
* We carefully and thoroughly flush databases upon shutdown and
  other important events already.