When importing a watch-only address over `importmulti` with a specific timestamp, the wallet's `nTimeFirstKey` will be set to 1.
I think we should add a optional timestamp to `CWallet::AddWatchOnly()` and use it when importing addresses or pubkeys.

{
    int nRequired = params[0].get_int();
    const UniValue& keys = params[1].get_array();
    
    if (nRequired < 1)
        throw runtime_error("a multisignature address must require at least one key to redeem");
    if ((int)keys.size() < nRequired)
        throw runtime_error(
            strprintf("not enough keys supplied "
                      "(got %u keys, but need at least %d to redeem)", keys.size(), nRequired));
    if (keys.size() > 16)
        throw runtime_error("Number of addresses involved in the multisignature address creation > 16\nReduce the number");
    std::vector<CPubKey> pubkeys;
    pubkeys.resize(keys.size());
    for (unsigned int i = 0; i < keys.size(); i++)
    {
        const std::string& ks = keys[i].get_str();
#ifdef ENABLE_WALLET
        
        CBitcoinAddress address(ks);
        if (pwalletMain && address.IsValid())
        {
            CKeyID keyID;
            if (!address.GetKeyID(keyID))
                throw runtime_error(
                    strprintf("%s does not refer to a key",ks));