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.

UniValue obj(UniValue::VOBJ);
        CScript subscript;
        obj.push_back(Pair("isscript", true));
        if (pwalletMain && pwalletMain->GetCScript(scriptID, subscript)) {
            std::vector<CTxDestination> addresses;
            txnouttype whichType;
            int nRequired;
            ExtractDestinations(subscript, whichType, addresses, nRequired);
            obj.push_back(Pair("script", GetTxnOutputType(whichType)));
            obj.push_back(Pair("hex", HexStr(subscript.begin(), subscript.end())));
            UniValue a(UniValue::VARR);
            BOOST_FOREACH(const CTxDestination& addr, addresses)
                a.push_back(CBitcoinAddress(addr).ToString());
            obj.push_back(Pair("addresses", a));
            if (whichType == TX_MULTISIG)
                obj.push_back(Pair("sigsrequired", nRequired));
        }
        return obj;
    }
};
#endif
UniValue validateaddress(const JSONRPCRequest& request)