message: [RPC] use json object as output of mintzerocoin
  txid and time fields can be grouped outside of the mints list as it is
  the same transaction.
diff: @@ -3342,17 +3342,20 @@ UniValue mintzerocoin(const UniValue& params, bool fHelp)
  " ]\n"
  "\nResult:\n"
  - "[\n"
  "{\n"
  " \"txid\": \"xxx\", (string) Transaction ID.\n"
  - " \"value\": amount, (numeric) Minted amount.\n"
  + " \"time\": nnn (numeric) Time to mint this transaction.\n"
  + " \"mints\":\n"
  + " [\n"
  + " {\n"
  + " \"denomination\": nnn, (numeric) Minted denomination.\n"
  " \"pubcoin\": \"xxx\", (string) Pubcoin in hex format.\n"
  " \"randomness\": \"xxx\", (string) Hex encoded randomness.\n"
  " \"serial\": \"xxx\", (string) Serial in hex format.\n"
  - " \"time\": nnn (numeric) Time to mint this transaction.\n"
  - " }\n"
  - " ,...\n"
  + " },\n"
  + " ...\n"
  " ]\n"
  + "}\n"
  "\nExamples:\n"
  "\nMint 50 from anywhere\n" +
  @@ -3420,20 +3423,22 @@ UniValue mintzerocoin(const UniValue& params, bool fHelp)
  if (strError != "")
  throw JSONRPCError(RPC_WALLET_ERROR, strError);
  + UniValue retObj(UniValue::VOBJ);
  + retObj.push_back(Pair("txid", wtx.GetHash().ToString()));
  + retObj.push_back(Pair("time", GetTimeMillis() - nTime));
  UniValue arrMints(UniValue::VARR);
  for (CDeterministicMint dMint : vDMints) {
  UniValue m(UniValue::VOBJ);
  - m.push_back(Pair("txid", wtx.GetHash().ToString()));
  - m.push_back(Pair("value", ValueFromAmount(libzerocoin::ZerocoinDenominationToAmount(dMint.GetDenomination()))));
  + m.push_back(Pair("denomination", ValueFromAmount(libzerocoin::ZerocoinDenominationToAmount(dMint.GetDenomination()))));
  m.push_back(Pair("pubcoinhash", dMint.GetPubcoinHash().GetHex()));
  m.push_back(Pair("serialhash", dMint.GetSerialHash().GetHex()));
  m.push_back(Pair("seedhash", dMint.GetSeedHash().GetHex()));
  m.push_back(Pair("count", (int64_t)dMint.GetCount()));
  - m.push_back(Pair("time", GetTimeMillis() - nTime));
  arrMints.push_back(m);
  }
  + retObj.push_back(Pair("mints", arrMints));
  - return arrMints;
  + return retObj;
  }
  UniValue spendzerocoin(const UniValue& params, bool fHelp)
  