Fix for 'bad encrypted message' errors.

1) There was a regression in the outgoing multipart transport
   logic, such that the same 'identifier' byte would be used
   for all messages (0).  This now works correctly.

2) Added some additional heuristics on the receiving side.
   Now mutlipart containers are only valid for 1hr, and are
   considered invalid if the container size is different from
   the multipart message size.