message: Update block header serialization
diff: @@ -149,10 +149,11 @@ public:
  READWRITE(mapAccumulatorHashes);
  READWRITE(hashMerkleRoot);
  READWRITE(hashWitnessMerkleRoot);
  + if (IsProofOfStake()) {
  READWRITE(hashPoFN);
  - if (IsProofOfStake())
  READWRITE(vchBlockSig);
  }
  + }
  void SetNull()
  {
  