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/folder/Maildir.py

@@ -60,7 +60,6 @@ class MaildirFolder(BaseFolder):
     def __init__(self, root, name, sep, repository):
         self.sep = sep # needs to be set before super().__init__
         super(MaildirFolder, self).__init__(name, repository)
-        self.dofsync = self.config.getdefaultboolean("general", "fsync", True)
         self.root = root
         # check if we should use a different infosep to support Win file systems
         self.wincompatible = self.config.getdefaultboolean(
@@ -330,7 +329,7 @@ class MaildirFolder(BaseFolder):
         fd.write(content)
         # Make sure the data hits the disk.
         fd.flush()
-        if self.dofsync:
+        if self.dofsync():
             os.fsync(fd)
         fd.close()