Split into separate encrypt and transmit functions. Let the encryption
function also handle all wire formatting (ie, jsonification and base64
encoding), which simplifes TextSecureServer.sendMessages, removes a
TODO, and lets us save fewer params to make network errors replayable.
// FREEBIE
1. Return the value returned by the registered function, to expose the
underlying promise to the caller.
2. Stop accepting extra arguments to the replay function. The caller
should be able to do what they want with the returned promise instead.
3. Add a timestamp argument to the outgoing case, needed to re-try
sending a message.
