async function getPositionUpdatesHandler(workspaceId, callback) {
  const positionUpdates = await database.getPositionUpdates(workspaceId);
  callback(positionUpdates);
}
Handler for a getPositionUpdates message. Query the database for a
PositionUpdate for the specified user or all if no user specified.
@param {string=} workspaceId The workspaceId for the specified user.
@param {!Function} callback The callback passed in by WorkspaceClient to
receive the PositionUpdates upon success of the query.
@private