Merge bitcoin/bitcoin#32740: refactor: Header sync optimisations & simplifications

de4242f474 refactor: Use reference for chain_start in HeadersSyncState (Daniela Brozzoni)
e37555e540 refactor: Use initializer list in CompressedHeader (Daniela Brozzoni)
0488bdfefe refactor: Remove unused parameter in ReportHeadersPresync (Daniela Brozzoni)
256246a9fa refactor: Remove redundant parameter from CheckHeadersPoW (Daniela Brozzoni)
ca0243e3a6 refactor: Remove useless CBlock::GetBlockHeader (Pieter Wuille)
4568652222 refactor: Use std::span in HasValidProofOfWork (Daniela Brozzoni)
4066bfe561 refactor: Compute work from headers without CBlockIndex (Daniela Brozzoni)
0bf6139e19 p2p: Avoid an IsAncestorOfBestHeaderOrTip call (Pieter Wuille)

Pull request description:

  This is a partial* revival of #25968

  It contains a list of most-unrelated simplifications and optimizations to the code merged in #25717:

  - Avoid an IsAncestorOfBestHeaderOrTip call: Just don't call this function when it won't have any effect.
  - Compute work from headers without CBlockIndex: Avoid the need to construct a CBlockIndex object just to compute work for a header, when its nBits value suffices for that. Also use some Spans where possible.
  - Remove useless CBlock::GetBlockHeader: There is no need for a function to convert a CBlock to a CBlockHeader, as it's a child class of it.

  It also contains the following code cleanups, which were suggested by reviewers in #25968:
  - Remove redundant parameter from CheckHeadersPoW: No need to pass consensusParams, as CheckHeadersPow already has access to m_chainparams.GetConsensus()
  - Remove unused parameter in ReportHeadersPresync
  - Use initializer list in CompressedHeader, also make GetFullHeader const
  - Use reference for chain_start in HeadersSyncState: chain_start can never be null, so it's better to pass it as a reference rather than a raw pointer

  *I decided to leave out three commits that were in #25968 (4e7ac7b94d04e056e9994ed1c8273c52b7b23931, ab52fb4e95aa2732d1a1391331ea01362e035984, 7f1cf440ca1a9c86085716745ca64d3ac26957c0), since they're a bit more involved, and I'm a new contributor. If this PR gets merged, I'll comment under #25968 to note that these three commits are still up for grabs :)

ACKs for top commit:
  l0rinc:
    ACK de4242f474
  polespinasa:
    re-ACK de4242f474
  sipa:
    ACK de4242f474
  achow101:
    ACK de4242f474
  hodlinator:
    re-ACK de4242f474

Tree-SHA512: 1de4f3ce0854a196712505f2b52ccb985856f5133769552bf37375225ea8664a3a7a6a9578c4fd461e935cd94a7cbbb08f15751a1da7651f8962c866146d9d4b

src/chain.cpp

@@ -118,12 +118,12 @@ void CBlockIndex::BuildSkip()
         pskip = pprev->GetAncestor(GetSkipHeight(nHeight));
 }
 
-arith_uint256 GetBlockProof(const CBlockIndex& block)
+arith_uint256 GetBitsProof(uint32_t bits)
 {
     arith_uint256 bnTarget;
     bool fNegative;
     bool fOverflow;
-    bnTarget.SetCompact(block.nBits, &fNegative, &fOverflow);
+    bnTarget.SetCompact(bits, &fNegative, &fOverflow);
     if (fNegative || fOverflow || bnTarget == 0)
         return 0;
     // We need to compute 2**256 / (bnTarget+1), but we can't represent 2**256