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.

obj.push_back(Pair("testnet",       Params().NetworkIDString() == CBaseChainParams::TESTNET));
#ifdef ENABLE_WALLET
    if (pwalletMain) {
        obj.push_back(Pair("keypoololdest", pwalletMain->GetOldestKeyPoolTime()));
        obj.push_back(Pair("keypoolsize",   (int)pwalletMain->GetKeyPoolSize()));
    }
    if (pwalletMain && pwalletMain->IsCrypted())
        obj.push_back(Pair("unlocked_until", nWalletUnlockTime));
    obj.push_back(Pair("paytxfee",      ValueFromAmount(payTxFee.GetFeePerK())));
#endif
    obj.push_back(Pair("relayfee",      ValueFromAmount(::minRelayTxFee.GetFeePerK())));
    obj.push_back(Pair("errors",        GetWarnings("statusbar")));
    return obj;
}
#ifdef ENABLE_WALLET
class DescribeAddressVisitor : public boost::static_visitor<UniValue>
{
public:
    UniValue operator()(const CNoDestination &dest) const { return UniValue(UniValue::VOBJ); }
    UniValue operator()(const CKeyID &keyID) const {
        UniValue obj(UniValue::VOBJ);
        CPubKey vchPubKey;
        obj.push_back(Pair("isscript", false));
        if (pwalletMain && pwalletMain->GetPubKey(keyID, vchPubKey)) {
            obj.push_back(Pair("pubkey", HexStr(vchPubKey)));
            obj.push_back(Pair("iscompressed", vchPubKey.IsCompressed()));
        }
        return obj;
    }
    UniValue operator()(const CScriptID &scriptID) const {