We are deprecating local_id/local_message_id on the Python server.
Instead of the server knowing about the client's implementation of
local id, with the message id = 9999.01 scheme, we just send the
server an opaque id to send back to us.
This commit changes the name from local_id -> client_message_id,
but it doesn't change the actual values passed yet.
The goal for client_key in future commits will be to:
* Have it for all messages, not just locally rendered messages
* Not have it overlap with server-side message ids.
The history behind local_id having numbers like 9999.01 is that
they are actually interim message ids and the numerical value is
used for rendering the message list when we do client-side rendering.
This reverts commit 7bf10ec74f.
Apparently, SockJS 1.1.1 is broken with the browser used in our legacy
desktop app, resulting in messages being silently not sent.
