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.

CPubKey vchPubKey;
            if (!pwalletMain->GetPubKey(keyID, vchPubKey))
                throw runtime_error(
                    strprintf("no full public key for address %s",ks));
            if (!vchPubKey.IsFullyValid())
                throw runtime_error(" Invalid public key: "+ks);
            pubkeys[i] = vchPubKey;
        }
        
        else
#endif
        if (IsHex(ks))
        {
            CPubKey vchPubKey(ParseHex(ks));
            if (!vchPubKey.IsFullyValid())
                throw runtime_error(" Invalid public key: "+ks);
            pubkeys[i] = vchPubKey;
        }
        else
        {
            throw runtime_error(" Invalid public key: "+ks);
        }
    }
    CScript result = GetScriptForMultisig(nRequired, pubkeys);
    if (result.size() > MAX_SCRIPT_ELEMENT_SIZE)
        throw runtime_error(
                strprintf("redeemScript exceeds size limit: %d > %d", result.size(), MAX_SCRIPT_ELEMENT_SIZE));
    return result;
}
UniValue createmultisig(const JSONRPCRequest& request)