Merge bitcoin/bitcoin#25717: p2p: Implement anti-DoS headers sync

3add23454624c4c79c9eebc060b6fbed4e3131a7 ui: show header pre-synchronization progress (Pieter Wuille)
738421c50f2dbd7395b50a5dbdf6168b07435e62 Emit NotifyHeaderTip signals for pre-synchronization progress (Pieter Wuille)
376086fc5a187f5b2ab3a0d1202ed4e6c22bdb50 Make validation interface capable of signalling header presync (Pieter Wuille)
93eae27031a65b4156df49015ae45b2b541b4e5a Test large reorgs with headerssync logic (Suhas Daftuar)
355547334f7d08640ee1fa291227356d61145d1a Track headers presync progress and log it (Pieter Wuille)
03712dddfbb9fe0dc7a2ead53c65106189f5c803 Expose HeadersSyncState::m_current_height in getpeerinfo() (Suhas Daftuar)
150a5486db50ff77c91765392149000029c8a309 Test headers sync using minchainwork threshold (Suhas Daftuar)
0b6aa826b53470c9cc8ef4a153fa710dce80882f Add unit test for HeadersSyncState (Suhas Daftuar)
83c6a0c5249c4ecbd11f7828c84a50fb473faba3 Reduce spurious messages during headers sync (Suhas Daftuar)
ed6cddd98e32263fc116a4380af6d66da20da990 Require callers of AcceptBlockHeader() to perform anti-dos checks (Suhas Daftuar)
551a8d957c4c44afbd0d608fcdf7c6a4352babce Utilize anti-DoS headers download strategy (Suhas Daftuar)
ed470940cddbeb40425960d51cefeec4948febe4 Add functions to construct locators without CChain (Pieter Wuille)
84852bb6bb3579e475ce78fe729fd125ddbc715f Add bitdeque, an std::deque<bool> analogue that does bit packing. (Pieter Wuille)
1d4cfa4272cf2c8b980cc8762c1ff2220d3e8d51 Add function to validate difficulty changes (Suhas Daftuar)

Pull request description:

  New nodes starting up for the first time lack protection against DoS from low-difficulty headers. While checkpoints serve as our protection against headers that fork from the main chain below the known checkpointed values, this protection only applies to nodes that have been able to download the honest chain to the checkpointed heights.

  We can protect all nodes from DoS from low-difficulty headers by adopting a different strategy: before we commit to storing a header in permanent storage, first verify that the header is part of a chain that has sufficiently high work (either `nMinimumChainWork`, or something comparable to our tip). This means that we will download headers from a given peer twice: once to verify the work on the chain, and a second time when permanently storing the headers.

  The p2p protocol doesn't provide an easy way for us to ensure that we receive the same headers during the second download of peer's headers chain. To ensure that a peer doesn't (say) give us the main chain in phase 1 to trick us into permanently storing an alternate, low-work chain in phase 2, we store commitments to the headers during our first download, which we validate in the second download.

  Some parameters must be chosen for commitment size/frequency in phase 1, and validation of commitments in phase 2. In this PR, those parameters are chosen to both (a) minimize the per-peer memory usage that an attacker could utilize, and (b) bound the expected amount of permanent memory that an attacker could get us to use to be well-below the memory growth that we'd get from the honest chain (where we expect 1 new block header every 10 minutes).

  After this PR, we should be able to remove checkpoints from our code, which is a nice philosophical change for us to make as well, as there has been confusion over the years about the role checkpoints play in Bitcoin's consensus algorithm.

  Thanks to Pieter Wuille for collaborating on this design.

ACKs for top commit:
  Sjors:
    re-tACK 3add23454624c4c79c9eebc060b6fbed4e3131a7
  mzumsande:
    re-ACK 3add23454624c4c79c9eebc060b6fbed4e3131a7
  sipa:
    re-ACK 3add23454624c4c79c9eebc060b6fbed4e3131a7
  glozow:
    ACK 3add234546

Tree-SHA512: e7789d65f62f72141b8899eb4a2fb3d0621278394d2d7adaa004675250118f89a4e4cb42777fe56649d744ec445ad95141e10f6def65f0a58b7b35b2e654a875

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