Update API docs and demos for recent worker improvements.

> You view it as "a huge maintainability burden", but I think it is a good fool-proof feature, and the maintenance cost is low, because GC does the heavy work for us.

Guaranteed correctness of GC imposes some restrictions on how we can develop zip.js, e.g. worker pooling. I don't want to limit our flexibility for the sake of those users who cannot call `.close()`.