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/LocalStatusSQLite.py

@@ -92,6 +92,20 @@ class LocalStatusSQLiteFolder(BaseFolder):
             LocalStatusSQLiteFolder.locks[self.filename] = DatabaseFileLock()
         self._databaseFileLock = LocalStatusSQLiteFolder.locks[self.filename]
 
+        self._in_transactions = 0
+
+    def __enter__(self):
+        if not self.dofsync():
+            assert self.connection is not None
+            self._in_transactions += 1
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if not self.dofsync():
+            assert self._in_transactions > 0
+            self._in_transactions -= 1
+            if self._in_transactions < 1:
+                self.connection.commit()
+
     def openfiles(self):
         # Make sure sqlite is in multithreading SERIALIZE mode.
         assert sqlite.threadsafety == 1, 'Your sqlite is not multithreading safe.'
@@ -169,7 +183,8 @@ class LocalStatusSQLiteFolder(BaseFolder):
                         else:
                             self.connection.execute(sql, args)
                     success = True
-                    self.connection.commit()
+                    if not self._in_transactions:
+                        self.connection.commit()
             except sqlite.OperationalError as e:
                 if e.args[0] == 'cannot commit - no transaction is active':
                     pass