The file-send protocol now sends a "hints-v1" key in the "transit"
message, which contains a list of JSON data structures that describe the
connection hints (a mixture of direct, tor, and relay hints, for now).
Previously the direct/tor and relay hints were sent in different keys,
and all were sent as strings like "tcp:hostname:1234" which had to be
parsed by the recipient.
The new structures include a version string, to make it easier to add
new types in the future. Transit logs+ignores hints it cannot
understand.
This adds an expected= argument to Connection.connectConsumer(), which
then returns a Deferred that fires when enough bytes have been written
to the consumer. It also adds Connection.writeToFile(), a helper method
that writes bytes to a filehandle.
I made the classic dataReceived() mistake, and exited the function after
delivering the first record. Keep at it until there are no complete
records left.
