Update API docs and demos for recent worker improvements.

1. If a library can't be proven to be implemented properly, don't use them, it can ruin users' data - much more dangerous than just leak memory. Whether a library prevents the workers from being GC'd  is also testable, just test it before using it.

2. `zip.closeReaderWorker()` just nullify the shared `zip.readerWorkder` without terminate it. You can setInterval() to call it. Exsiting ZipReaders can still use the old worker until death, and newly created ZipReaders will get a new worker in `createZipReader()`.