ImageWorker improvements

[incognitojam]

• [ ] speed up by fetching, unpacking and verifying checksum at the same time instead of sequentially
  • [x] partial speed-up in https://github.com/commaai/flash/pull/16
• [ ] store and re-use images from the filesystem (unpack image from disk and verify the checksum, deleting it and re-fetching if the checksum is wrong)

CI takes around 6.5 minutes to download/unpack/verify every image, so the web flasher should try to get close to this