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.h

@@ -340,6 +340,12 @@ bool TestBlockValidity(BlockValidationState& state,
                        bool fCheckPOW = true,
                        bool fCheckMerkleRoot = true) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
+/** Check with the proof of work on each blockheader matches the value in nBits */
+bool HasValidProofOfWork(const std::vector<CBlockHeader>& headers, const Consensus::Params& consensusParams);
+
+/** Return the sum of the work on a given set of headers */
+arith_uint256 CalculateHeadersWork(const std::vector<CBlockHeader>& headers);
+
 /** RAII wrapper for VerifyDB: Verify consistency of the block and coin databases */
 class CVerifyDB {
 public: