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() != 2)
        throw runtime_error(
            "importprunedfunds\n"
            "\nImports funds without rescan. Corresponding address or script must previously be included in wallet. Aimed towards pruned wallets. The end-user is responsible to import additional transactions that subsequently spend the imported outputs or rescan after the point in the blockchain the transaction is included.\n"
            "\nArguments:\n"
            "1. \"rawtransaction\" (string, required) A raw transaction in hex funding an already-existing address in wallet\n"
            "2. \"txoutproof\"     (string, required) The hex output from gettxoutproof that contains the transaction\n"
        );
    CMutableTransaction tx;
    if (!DecodeHexTx(tx, request.params[0].get_str()))
        throw JSONRPCError(RPC_DESERIALIZATION_ERROR, "TX decode failed");
    uint256 hashTx = tx.GetHash();
    CWalletTx wtx(pwalletMain, MakeTransactionRef(std::move(tx)));