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.

CBitcoinAddress address;
    if (!address.SetString(strAddress))
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid Bitcoin address");
    CKeyID keyID;
    if (!address.GetKeyID(keyID))
        throw JSONRPCError(RPC_TYPE_ERROR, "Address does not refer to a key");
    CKey vchSecret;
    if (!pwalletMain->GetKey(keyID, vchSecret))
        throw JSONRPCError(RPC_WALLET_ERROR, "Private key for address " + strAddress + " is not known");
    return CBitcoinSecret(vchSecret).ToString();
}
UniValue dumpwallet(const JSONRPCRequest& request)
{
    if (!EnsureWalletIsAvailable(request.fHelp))
        return NullUniValue;
    
    if (request.fHelp || request.params.size() != 1)
        throw runtime_error(
            "dumpwallet \"filename\"\n"
            "\nDumps all wallet keys in a human-readable format.\n"
            "\nArguments:\n"
            "1. \"filename\"    (string, required) The filename\n"
            "\nExamples:\n"
            + HelpExampleCli("dumpwallet", "\"test\"")
            + HelpExampleRpc("dumpwallet", "\"test\"")
        );
    LOCK2(cs_main, pwalletMain->cs_wallet);
    EnsureWalletIsUnlocked();
    ofstream file;
    file.open(request.params[0].get_str().c_str());