message: validation: Use accessible chainstate in ChainstateManager::ProcessNewBlock
  [META] This commit should be followed up by removing the comments and
  assertions meant only to show that the change is correct.
diff: @@ -3660,6 +3660,7 @@ bool CChainState::AcceptBlock(const std::shared_ptr<const CBlock>& pblock, Block
  bool ChainstateManager::ProcessNewBlock(const CChainParams& chainparams, const std::shared_ptr<const CBlock> pblock, bool fForceProcessing, bool* fNewBlock, CBlockIndex** ppindex, bool* fPoSDuplicate)
  {
  AssertLockNotHeld(cs_main);
  + assert(std::addressof(::ChainstateActive()) == std::addressof(ActiveChainstate()));
  {
  CBlockIndex *pindex = nullptr;
  @@ -3676,7 +3677,7 @@ bool ChainstateManager::ProcessNewBlock(const CChainParams& chainparams, const s
  bool ret = CheckBlock(*pblock, state, chainparams.GetConsensus());
  if (ret) {
  // Store to disk
  - ret = ::ChainstateActive().AcceptBlock(pblock, state, chainparams, &pindex, fForceProcessing, nullptr, fNewBlock);
  + ret = ActiveChainstate().AcceptBlock(pblock, state, chainparams, &pindex, fForceProcessing, nullptr, fNewBlock);
  }
  if (ppindex)
  *ppindex = ret ? pindex : nullptr;
  @@ -3693,10 +3694,10 @@ bool ChainstateManager::ProcessNewBlock(const CChainParams& chainparams, const s
  }
  }
  - NotifyHeaderTip(::ChainstateActive());
  + NotifyHeaderTip(ActiveChainstate());
  BlockValidationState state; // Only used to report errors, not invalidity - ignore it
  - if (!::ChainstateActive().ActivateBestChain(state, chainparams, pblock))
  + if (!ActiveChainstate().ActivateBestChain(state, chainparams, pblock))
  return error("%s: ActivateBestChain failed (%s)", __func__, state.ToString());
  return true;
  