function forever(fn, errback) {
    var done = onlyOnce(errback || noop);
    var task = wrapAsync(ensureAsync(fn));

    function next(err) {
        if (err) return done(err);
        task(next);
    }
    next();
}
Calls the asynchronous function `fn` with a callback parameter that allows it
to call itself again, in series, indefinitely.
If an error is passed to the callback then `errback` is called with the
error, and execution stops, otherwise it will never be called.

@name forever
@static
@memberOf module:ControlFlow
@method
@category Control Flow
@param {AsyncFunction} fn - an async function to call repeatedly.
Invoked with (next).
@param {Function} [errback] - when `fn` passes an error to it's callback,
this function will be called, and execution stops. Invoked with (err).
@example

async.forever(
    function(next) {
        // next is suitable for passing to things that need a callback(err [, whatever]);
        // it will result in this function being called again.
    },
    function(err) {
        // if next is called with a value in its first parameter, it will appear
        // in here as 'err', and execution will stop.
    }
);