const BLOCK_DOWNLOAD_RETRY_LIMIT: usize = 3;
Controls how many times we will retry each block download.
Failing block downloads is important because it defends against peers who feed us bad hashes. But spurious failures of valid blocks cause the syncer to restart from the previous checkpoint, potentially re-downloading blocks.
We also hedge requests, so we may retry up to twice this many times. Hedged retries may be concurrent, inner retries are sequential.