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.

{
    if (!EnsureWalletIsAvailable(request.fHelp))
        return NullUniValue;
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 4)
        throw runtime_error(
            "importpubkey \"pubkey\" ( \"label\" rescan )\n"
            "\nAdds a public key (in hex) that can be watched as if it were in your wallet but cannot be used to spend.\n"
            "\nArguments:\n"
            "1. \"pubkey\"           (string, required) The hex-encoded public key\n"
            "2. \"label\"            (string, optional, default=\"\") An optional label\n"
            "3. rescan               (boolean, optional, default=true) Rescan the wallet for transactions\n"
            "\nNote: This call can take minutes to complete if rescan is true.\n"
            "\nExamples:\n"
            "\nImport a public key with rescan\n"
            + HelpExampleCli("importpubkey", "\"mypubkey\"") +
            "\nImport using a label without rescan\n"