learn singlethreadperfolder configuration option

To further ensure that messages are synchronized strictly in UID
order, this option can be set to cause only one thread to be used
to synchronise an individual folder, though other folders may
be synchronized simultaneously by other threads.

Signed-off-by: James E. Blair <corvus@gnu.org>
Based-on-patch-by: James E. Blair <corvus@gnu.org>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/IMAP.py

@@ -79,6 +79,11 @@ class IMAPFolder(BaseFolder):
 
     # Interface from BaseFolder
     def suggeststhreads(self):
+        onethread = self.config.getdefaultboolean(
+            "Repository %s"% self.repository.getname(),
+            "singlethreadperfolder", False)
+        if onethread is True:
+            return False
         return not globals.options.singlethreading
 
     # Interface from BaseFolder