[Bugfix/refinement] ota-proxy timeout issue (#183)
* fix and refine OngoingCacheTracker, resolve the unwanted timeout issue, resolve the edge condition that subscriber subscribes on closed tracker,
* move more configuration to ota_proxy.config instead of hard-coded,
* fix and refine the RemoteOTAFile, simplify the background_cache_writing logic and cache commit logic,
* refine OTACache.retrieve_file,
* deps: bump uvicorn==0.20.0, aiofiles==22.1.0, and pin aiohttp==3.8.1,
* optimize ota_proxy configuration according to test result,
* test files updates,
* MAX_DOWNLOAD_THREAD reduces from 8 to 3, as the benefit is not significant,
* use uvloop for ota-proxy, which results in around 10%~ or more performance improved,
* reduce CHUNK_SIZE to 1MB, tests reveal that this setting has better performance,
* reduce STREAMING_BACKOFF_FACTOR to 0.001 to reduce blocking,
* DB_THREAD_POOL_SIZE set to 1
