SQLite: make postponing transaction committing possible.

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>

offlineimap.conf

@@ -164,6 +164,9 @@ accounts = Test
 # at the expense of greater risk of message duplication in the event of a system
 # crash or power loss.  Default is true.  Set it to false to disable fsync.
 #
+# SQLite honors this option since v7.0.8+. However, those SQLite improvements
+# are still EXPERIMENTAL.
+#
 #fsync = true