Speed up Flash

- [ ] The download of images from Azure is slow - takes about 2.5 min for 900MB system image (today 4.5m...)
- [x] download/unpack in parallel (if both download and unpack take 4m, we have saved 4m)
- [x] replace `js-sha` with [`hash-wasm`](https://www.npmjs.com/package/hash-wasm) (unpack 10m -> 4m)
- updating the sha256 hash for each chunk during unpack is slow - seems to take 6ms for every chunk
- [ ] `xz-decompress` is slow
- [ ] There may be overhead because for the XZ decompression and SHA calculation we transfer bytes from JS to WASM (and back for decompression)... Not sure yet how complex it is to write something to process everything on the WASM side.
- Moving the `QdlManager` to a web worker (instead of just `ImageWorker`) may also help speed things up as it would move the flashing (which includes computations like `Sparse.splitBlob`) off of the main/render thread
- Faster flashing of sparse images in `qdl.js` (flashing from 2:30 to 1:50)
- [x] fast erase partition/sector
- [x] skip chunks during flashing
- [ ] Stream images to flash while downloading (`qdl.js` needs to accept a `ReadableStream` for blob to flash)

You can generate perf traces in Chrome in the performance tab

Start with qdl.js, QdlManager, ImageWorker, and Sparse.splitBlob, then use Chrome's Performance tab to reproduce the reported flashing and download timings. The issue lists several possible optimization paths, including streaming downloads and moving work off the render thread; done would require selecting and implementing a scoped improvement with measurable performance gains.

Written by the indexing model from the issue text.