We need this method for further synchronization: once this is sent we
expect the traders to respond with signatures for each of the accounts
involved in the batch. The addition of this message also means that the
responder of the channel funding has time to register their shim (done
before they send their Accept message) as we don't send the
OrderSignBegin message until all traders have accepted the batch. The
initiator can now use receipt of the OrderSignBegin as a signal to
initiate the channel funding.
We also fix an existing oversight in the `OrderMatchSign`: it expected a
fully valid witness, but the only thing the traders can produce by
themselves is a signature for their input.
