message: Add minimum zPIV maturity consideration to GetMintMaturityHeight().
  The zPIV should not be considered mature unless it has 1 mint that occured after it AND the mint is at least two checkpoints deep.
diff: @@ -514,6 +514,9 @@ map<CoinDenomination, int> GetMintMaturityHeight()
  mapDenomMaturity.insert(make_pair(denom, make_pair(0, 0)));
  int nConfirmedHeight = chainActive.Height() - Params().Zerocoin_MintRequiredConfirmations();
  +
  + // A mint need to get to at least the min maturity height before it will spend.
  + int nMinimumMaturityHeight = nConfirmedHeight - (nConfirmedHeight % 10);
  CBlockIndex* pindex = chainActive[nConfirmedHeight];
  while (pindex && pindex->nHeight > Params().Zerocoin_StartHeight()) {
  @@ -526,7 +529,7 @@ map<CoinDenomination, int> GetMintMaturityHeight()
  //if mint was found then record this block as the first block that maturity occurs.
  if (mapDenomMaturity.at(denom).first >= Params().Zerocoin_RequiredAccumulation())
  - mapDenomMaturity.at(denom).second = pindex->nHeight;
  + mapDenomMaturity.at(denom).second = std::min(pindex->nHeight, nMinimumMaturityHeight);
  //Signal that we are finished
  isFinished = false;
  