Utilize anti-DoS headers download strategy

Avoid permanently storing headers from a peer, unless the headers are part of a
chain with sufficiently high work. This prevents memory attacks using low-work
headers.

Designed and co-authored with Pieter Wuille.

src/validation.cpp

@@ -3432,6 +3432,22 @@ std::vector<unsigned char> ChainstateManager::GenerateCoinbaseCommitment(CBlock&
     return commitment;
 }
 
+bool HasValidProofOfWork(const std::vector<CBlockHeader>& headers, const Consensus::Params& consensusParams)
+{
+    return std::all_of(headers.cbegin(), headers.cend(),
+            [&](const auto& header) { return CheckProofOfWork(header.GetHash(), header.nBits, consensusParams);});
+}
+
+arith_uint256 CalculateHeadersWork(const std::vector<CBlockHeader>& headers)
+{
+    arith_uint256 total_work{0};
+    for (const CBlockHeader& header : headers) {
+        CBlockIndex dummy(header);
+        total_work += GetBlockProof(dummy);
+    }
+    return total_work;
+}
+
 /** Context-dependent validity checks.
  *  By "context", we mean only the previous block headers, but not the UTXO
  *  set; UTXO-related validity checks are done in ConnectBlock().