[mboximport] import the mail box messages sorted by date (closes #2515093)

This is required since the EmailThread creation logic strongly rely on the order which Email entities are created

mboximport.py

@@ -64,5 +64,6 @@
         self.skipped.append(messageid)
 
     def import_mbox_stream(self, stream):
-        from mailbox import UnixMailbox
+        from mailbox import UnixMailbox as Mailbox
+        from rfc822 import parsedate
         from logilab.common.umessage import message_from_file
@@ -68,5 +69,6 @@
         from logilab.common.umessage import message_from_file
-        for message in UnixMailbox(stream, message_from_file):
+        for message in sorted(Mailbox(stream, message_from_file),
+                              key=lambda x:parsedate(x['Date'])):
             try:
                 self.import_message(message)
                 if self.autocommit: