This should significantly improve performance when used to write large
amounts of messages.
This feature is enabled through the fsync configuration option.
Code refactorize around fsync.
This addresses #390 (although it doesn't necessarily fix all instances
of that problem yet).
Github-ref: https://github.com/OfflineIMAP/offlineimap/issues/390
Originally-written-by: Giel van Schijndel <me@mortis.eu>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>