async function updatePositionHandler(user, positionUpdate, callback) {
  await database.updatePosition(positionUpdate);
  callback();
  user.broadcast.emit('broadcastPosition', [positionUpdate]);
}
Handler for an updatePosition message. Update a user's position in the
users table and broadcast the PositionUpdate to all users except the
sender.
@param {!Object} user The user who sent the message.
@param {!PositionUpdate} positionUpdate The PositionUpdate with the new
position for a given user.
@param {!Function} callback The callback passed in by WorkspaceClient to
receive acknowledgement of the success of the write.
@private