message: Rename the flags to proof type
  The flags contained more info when using stakemodifier v1, when switch to stakemodifier v2, it only present the proof type.
diff: @@ -204,11 +204,7 @@ public:
  uint256 hashStateRoot; // qtum
  // block signature - proof-of-stake protect the block by signing the block using a stake holder private key
  std::vector<unsigned char> vchBlockSig;
  - unsigned int nFlags; // ppcoin: block index flags
  - enum
  - {
  - BLOCK_PROOF_OF_STAKE = (1 << 0), // is proof-of-stake block
  - };
  + bool fStake;
  uint256 nStakeModifier;
  // proof-of-stake specific fields
  COutPoint prevoutStake;
  @@ -245,7 +241,7 @@ public:
  nNonce = 0;
  hashStateRoot = uint256(); // qtum
  vchBlockSig.clear();
  - nFlags = 0;
  + fStake = 0;
  nStakeModifier = uint256();
  hashProof = uint256();
  prevoutStake.SetNull();
  @@ -267,7 +263,7 @@ public:
  nBits = block.nBits;
  nNonce = block.nNonce;
  hashStateRoot = block.hashStateRoot; // qtum
  - nFlags = 0;
  + fStake = 0;
  nStakeModifier = uint256();
  hashProof = uint256();
  if (block.IsProofOfStake())
  @@ -353,17 +349,17 @@ public:
  bool IsProofOfWork() const // qtum
  {
  - return !(nFlags & BLOCK_PROOF_OF_STAKE);
  + return !fStake;
  }
  bool IsProofOfStake() const
  {
  - return (nFlags & BLOCK_PROOF_OF_STAKE);
  + return fStake;
  }
  void SetProofOfStake() // qtum
  {
  - nFlags |= BLOCK_PROOF_OF_STAKE;
  + fStake = true;
  }
  std::string ToString() const
  @@ -449,7 +445,7 @@ public:
  READWRITE(nBits);
  READWRITE(nNonce);
  READWRITE(hashStateRoot); // qtum
  - READWRITE(nFlags);
  + READWRITE(fStake);
  READWRITE(nStakeModifier);
  if (IsProofOfStake())
  {
  @@ -476,7 +472,7 @@ public:
  block.nNonce = nNonce;
  block.hashStateRoot = hashStateRoot; // qtum
  block.vchBlockSig = vchBlockSig;
  - block.fStake = IsProofOfStake();
  + block.fStake = fStake;
  block.prevoutStake = prevoutStake;
  block.nStakeTime = nStakeTime; // qtum
  return block.GetHash();
  