send-twisted: set transit key before sending phase1
send-twisted: set transit key before sending phase1

This ensures that we'll be ready for them. Previously there was a race
between us revealing the direct hints to the peer, and us setting the
transit key (thus allowing us to check inbound handshake requests). The
Transit instance didn't handle the race, causing errors to be thrown
when the other side connected quickly.

warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole,warner/magic-wormhole