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() != 1)
        throw runtime_error(
            "validateaddress \"address\"\n"
            "\nReturn information about the given bitcoin address.\n"
            "\nArguments:\n"
            "1. \"address\"     (string, required) The bitcoin address to validate\n"
            "\nResult:\n"
            "{\n"
            "  \"isvalid\" : true|false,       (boolean) If the address is valid or not. If not, this is the only property returned.\n"