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.

{
    /* Please, avoid using the word "pool" here in the RPC interface or help,
     * as users will undoubtedly confuse it with the other "memory pool"
     */
    if (request.fHelp || request.params.size() != 0)
        throw runtime_error(
            "getmemoryinfo\n"
            "Returns an object containing information about memory usage.\n"
            "\nResult:\n"
            "{\n"
            "  \"locked\": {               (json object) Information about locked memory manager\n"
            "    \"used\": xxxxx,          (numeric) Number of bytes used\n"
            "    \"free\": xxxxx,          (numeric) Number of bytes available in current arenas\n"
            "    \"total\": xxxxxxx,       (numeric) Total number of bytes managed\n"
            "    \"locked\": xxxxxx,       (numeric) Amount of bytes that succeeded locking. If this number is smaller than total, locking pages failed at some point and key data could be swapped to disk.\n"
            "    \"chunks_used\": xxxxx,   (numeric) Number allocated chunks\n"
            "    \"chunks_free\": xxxxx,   (numeric) Number unused chunks\n"
            "  }\n"
            "}\n"
            "\nExamples:\n"
            + HelpExampleCli("getmemoryinfo", "")
            + HelpExampleRpc("getmemoryinfo", "")
        );
    UniValue obj(UniValue::VOBJ);
    obj.push_back(Pair("locked", RPCLockedMemoryInfo()));
    return obj;
}
UniValue echo(const JSONRPCRequest& request)