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 (request.fHelp || request.params.size() != 2)
        throw runtime_error(
            "signmessagewithprivkey \"privkey\" \"message\"\n"
            "\nSign a message with the private key of an address\n"
            "\nArguments:\n"
            "1. \"privkey\"         (string, required) The private key to sign the message with.\n"
            "2. \"message\"         (string, required) The message to create a signature of.\n"
            "\nResult:\n"
            "\"signature\"          (string) The signature of the message encoded in base 64\n"
            "\nExamples:\n"
            "\nCreate the signature\n"
            + HelpExampleCli("signmessagewithprivkey", "\"privkey\" \"my message\"") +
            "\nVerify the signature\n"
            + HelpExampleCli("verifymessage", "\"1D1ZrZNe3JUo7ZycKEYQQiQAWd9y54F4XZ\" \"signature\" \"my message\"") +
            "\nAs json rpc\n"
            + HelpExampleRpc("signmessagewithprivkey", "\"privkey\", \"my message\"")
        );
    string strPrivkey = request.params[0].get_str();
    string strMessage = request.params[1].get_str();
    CBitcoinSecret vchSecret;
    bool fGood = vchSecret.SetString(strPrivkey);
    if (!fGood)
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Invalid private key");
    CKey key = vchSecret.GetKey();
    if (!key.IsValid())
        throw JSONRPCError(RPC_INVALID_ADDRESS_OR_KEY, "Private key outside allowed range");
    CHashWriter ss(SER_GETHASH, 0);