Option to enable fsync on writes

[ato]

By default RocksDB writes batches asynchronously. This means in a power loss scenario, it's possible for recently committed writes to be lost. This may or may not be acceptable depending on how you use OutbackCDX. Also depending on your hardware (e.g. battery-backed write caches) fsync can be either quite slow or quite fast so we probably want to default to sync for safety but make it configurable.