Update API docs and demos for recent worker improvements.

```
// release the worker if no ZipReaders have been created for 5 minutes. 
setInterval(zip.releaseSharedReaderWorker, 300 * 1000);
```

This API design is atrocious...!
I don't see the benefit of introducing a NEW destruction method instead of recommending to use `.close()`. If the user forget to call `.releaseSharedReaderWorker`, or if the user forgets to clear the timer, then the zip API falls apart.