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.

"  \"paytxfee\": x.xxxx,         (numeric) the transaction fee set in " + CURRENCY_UNIT + "/kB\n"
            "  \"relayfee\": x.xxxx,         (numeric) minimum relay fee for non-free transactions in " + CURRENCY_UNIT + "/kB\n"
            "  \"errors\": \"...\"           (string) any error messages\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("getinfo", "")
            + HelpExampleRpc("getinfo", "")
        );
#ifdef ENABLE_WALLET
    LOCK2(cs_main, pwalletMain ? &pwalletMain->cs_wallet : NULL);
#else
    LOCK(cs_main);
#endif
    proxyType proxy;
    GetProxy(NET_IPV4, proxy);
    UniValue obj(UniValue::VOBJ);
    obj.push_back(Pair("version", CLIENT_VERSION));
    obj.push_back(Pair("protocolversion", PROTOCOL_VERSION));
#ifdef ENABLE_WALLET
    if (pwalletMain) {
        obj.push_back(Pair("walletversion", pwalletMain->GetVersion()));
        obj.push_back(Pair("balance",       ValueFromAmount(pwalletMain->GetBalance())));
    }
#endif
    obj.push_back(Pair("blocks",        (int)chainActive.Height()));
    obj.push_back(Pair("timeoffset",    GetTimeOffset()));
    if(g_connman)
        obj.push_back(Pair("connections",   (int)g_connman->GetNodeCount(CConnman::CONNECTIONS_ALL)));
    obj.push_back(Pair("proxy",         (proxy.IsValid() ? proxy.proxy.ToStringIPPort() : string())));
    obj.push_back(Pair("difficulty",    (double)GetDifficulty()));