libtransport: actually send people history on rejoin!

- History is now actually replayed when you rejoin a MUC, which is good.
- Here's a fun code smell: Frontend::sendMessage() *needs* a shared_ptr
  to the message, although it should really be a reference. That's
  something to fix in a later change.
- If you're joining a *new* device, you won't get any history. I'm not
  sure whether this behaviour is a good idea or not.
- Also, if spectrum2 crashes you're still going to miss messages, since
  it doesn't attempt to rejoin you to your channels if such a thing
  occurs.