- The new CacheWriter is simplified somewhat
- Blocking on PriorityTaskManager.proceed is moved out of
CacheWriter and into the Downloader tasks. This is because
we want to shift only the caching parts of the Downloaders
onto their Executors, whilst keeping the blocking parts on
the main Downloader threads. Else we can end up "using"
the Executor threads indefinitely whilst they're blocked.
Issue: #5978
PiperOrigin-RevId: 313222923
d88f5f47e6