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/UIDMaps.py

@@ -34,7 +34,6 @@ class MappedIMAPFolder(IMAPFolder):
 
     Instance variables (self.):
       dryrun: boolean.
-      dofsync: boolean for fsync calls.
       r2l: dict mapping message uids: self.r2l[remoteuid]=localuid
       l2r: dict mapping message uids: self.r2l[localuid]=remoteuid
       #TODO: what is the difference, how are they used?
@@ -44,7 +43,6 @@ class MappedIMAPFolder(IMAPFolder):
     def __init__(self, *args, **kwargs):
         IMAPFolder.__init__(self, *args, **kwargs)
         self.dryrun = self.config.getdefaultboolean("general", "dry-run", True)
-        self.dofsync = self.config.getdefaultboolean("general", "fsync", True)
         self.maplock = Lock()
         self.diskr2l, self.diskl2r = self._loadmaps()
         self.r2l, self.l2r = None, None
@@ -114,7 +112,7 @@ class MappedIMAPFolder(IMAPFolder):
             with open(mapfilenametmp, 'wt') as mapfilefd:
                 for (key, value) in self.diskl2r.items():
                     mapfilefd.write("%d:%d\n"% (key, value))
-                if self.dofsync is True:
+                if self.dofsync():
                     fsync(mapfilefd)
             # The lock is released when the file descriptor ends.
             shutil.move(mapfilenametmp, mapfilename)