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.

* and is thus planned to be deprecated eventually.
 *
 * Based on the source of the information, new information should be added to:
 * - `getblockchaininfo`,
 * - `getnetworkinfo` or
 * - `getwalletinfo`
 *
 * Or alternatively, create a specific query method for the information.
 **/
UniValue getinfo(const JSONRPCRequest& request)
{
    if (request.fHelp || request.params.size() != 0)
        throw runtime_error(
            "getinfo\n"
            "\nDEPRECATED. Returns an object containing various state info.\n"
            "\nResult:\n"
            "{\n"
            "  \"version\": xxxxx,           (numeric) the server version\n"
            "  \"protocolversion\": xxxxx,   (numeric) the protocol version\n"
            "  \"walletversion\": xxxxx,     (numeric) the wallet version\n"
            "  \"balance\": xxxxxxx,         (numeric) the total bitcoin balance of the wallet\n"
            "  \"blocks\": xxxxxx,           (numeric) the current number of blocks processed in the server\n"
            "  \"timeoffset\": xxxxx,        (numeric) the time offset\n"
            "  \"connections\": xxxxx,       (numeric) the number of connections\n"
            "  \"proxy\": \"host:port\",     (string, optional) the proxy used by the server\n"
            "  \"difficulty\": xxxxxx,       (numeric) the current difficulty\n"
            "  \"testnet\": true|false,      (boolean) if the server is using testnet or not\n"
            "  \"keypoololdest\": xxxxxx,    (numeric) the timestamp (seconds since Unix epoch) of the oldest pre-generated key in the key pool\n"
            "  \"keypoolsize\": xxxx,        (numeric) how many new keys are pre-generated\n"
            "  \"unlocked_until\": ttt,      (numeric) the timestamp in seconds since epoch (midnight Jan 1 1970 GMT) that the wallet is unlocked for transfers, or 0 if the wallet is locked\n"