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 handles this race (with an interlock on the transit
key), but it's still nicer to do it cleanly.
This exposed a new race in Transit, where the inbound connection would
complete before transit.connect() had been called. The previous commit
adds an interlock to wait for that too. Until this change, the transit
key lock was covering that one up.
