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.

"  \"address\" : \"address\", (string) The bitcoin address validated\n"
            "  \"scriptPubKey\" : \"hex\",       (string) The hex encoded scriptPubKey generated by the address\n"
            "  \"ismine\" : true|false,        (boolean) If the address is yours or not\n"
            "  \"iswatchonly\" : true|false,   (boolean) If the address is watchonly\n"
            "  \"isscript\" : true|false,      (boolean) If the key is a script\n"
            "  \"pubkey\" : \"publickeyhex\",    (string) The hex value of the raw public key\n"
            "  \"iscompressed\" : true|false,  (boolean) If the address is compressed\n"
            "  \"account\" : \"account\"         (string) DEPRECATED. The account associated with the address, \"\" is the default account\n"
            "  \"timestamp\" : timestamp,        (number, optional) The creation time of the key if available in seconds since epoch (Jan 1 1970 GMT)\n"
            "  \"hdkeypath\" : \"keypath\"       (string, optional) The HD keypath if the key is HD and available\n"
            "  \"hdmasterkeyid\" : \"<hash160>\" (string, optional) The Hash160 of the HD master pubkey\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("validateaddress", "\"1PSSGeFHDnKNxiEyFrD1wcEaHr9hrQDDWc\"")
            + HelpExampleRpc("validateaddress", "\"1PSSGeFHDnKNxiEyFrD1wcEaHr9hrQDDWc\"")
        );
#ifdef ENABLE_WALLET
    LOCK2(cs_main, pwalletMain ? &pwalletMain->cs_wallet : NULL);
#else
    LOCK(cs_main);
#endif
    CBitcoinAddress address(request.params[0].get_str());
    bool isValid = address.IsValid();
    UniValue ret(UniValue::VOBJ);
    ret.push_back(Pair("isvalid", isValid));
    if (isValid)
    {
        CTxDestination dest = address.Get();
        string currentAddress = address.ToString();
        ret.push_back(Pair("address", currentAddress));