feat: avoid explosion of disordering blocks based on BLOCK_DOWNLOAD_WINDOW

[keroro520]

The "sync" protocol downloads different blocks from multiple peers. In the case of different download speeds between peers, that will cause some disordering blocks backlog in orphan_pool.

To avoid the explosion of disordering blocks, we limit the download speed based on BLOCK_DOWNLOAD_WINDOW:

// Size of the "block download window": how far ahead of our current height do we fetch?
// Larger windows tolerate larger download speed differences between peers, but increase the
// potential degree of disordering of blocks.
pub const BLOCK_DOWNLOAD_WINDOW: u64 = 1024 * 8; // 1024 * default_outbound_peers

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BLOCK_DOWNLOAD_WINDOW should be good enough to limit the disordering blocks but meanwhile, tolerate download speed differences between peers. I am not sure whether 1024 * 8 is good enough, but it is a safe value in my opinion.

[driftluo]

Only after asynchronous verification will it work here.

[driftluo]

bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push