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f6a9166 Add functional test for block announcements during initial headers sync (Suhas Daftuar)
05f7f31 Reduce bandwidth during initial headers sync when a block is found (Suhas Daftuar)

Pull request description:

  On startup, if our headers chain is more than a day behind current time, we'll pick one peer to sync headers with until our best headers chain is caught up (at that point, we'll try to sync headers with all peers).

  However, if an INV for a block is received before our headers chain is caught up, we'll then start to sync headers from each peer announcing the block.  This can result in doing a big headers sync with many (if not all) of our peers simultaneously, which wastes bandwidth.

  This PR would reduce that overhead by picking (at most) one new peer to try syncing headers with whenever a new block is announced, prior to our headers chain being caught up.

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Tree-SHA512: 0662000bd68db146f55981de4adc2e2b07cbfda222b1176569d61c22055e5556752ffd648426f69687ed1cc203105515e7304c12b915d6270df8e41a4a0e1eaa


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Bitcoin Core integration/staging tree

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Bitcoin Core connects to the Bitcoin peer-to-peer network to download and fully validate blocks and transactions. It also includes a wallet and graphical user interface, which can be optionally built.

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