add new config option filename_use_mail_timestamp

If this value is true, use (if possible) a timestamp based on message
Date or Delivery-date headers. The current system time is used
otherwise.

filename_use_mail_timestamp and utime_from_header are now completely
separated option that do not interfere one with other.

To handle this feature in a multithread context we use a hash to count
the number of mail with the same timestamp. This method is more accurate
than using the old lasttime and timeseq variables.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/Base.py

@@ -60,6 +60,13 @@ class BaseFolder(object):
         self._utime_from_header = self.config.getdefaultboolean(repo,
             "utime_from_header", utime_from_header_global)
 
+        # Do we need to use mail timestamp for filename prefix?
+        filename_use_mail_timestamp_global = self.config.getdefaultboolean(
+            "general", "filename_use_mail_timestamp", False)
+        repo = "Repository " + repository.name
+        self._filename_use_mail_timestamp = self.config.getdefaultboolean(repo,
+            "filename_use_mail_timestamp", filename_use_mail_timestamp_global)
+
         # Determine if we're running static or dynamic folder filtering
         # and check filtering status
         self._dynamic_folderfilter = self.config.getdefaultboolean(