Correlate /v1/zwrite calls with the message stream

[davidben]

In a similar vein to #39. This would want either a client-allocated identifier (ow) or for more API calls to move to the socket so that their responses can be ordered relative to /v1/zwrite responses.

[davidben]

Simpler thing to start with that would get most of the way there: store the UID in the database and send it back. Also send the UID back in response to /v1/zwrite.

More complex thing for future: socket-based zwrite call. Sends back two responses. First the UID which is available synchronously and can have ordering guarantees with the tails. Then it sends a second message when the SERVACK comes back.