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.

{
    LOCK(cs_wallet);
    
    if (setKeyPool.empty())
        return GetTime();
    
    CKeyPool keypool;
    CWalletDB walletdb(strWalletFile);
    int64_t nIndex = *(setKeyPool.begin());
    if (!walletdb.ReadPool(nIndex, keypool))
        throw runtime_error(std::string(__func__) + ": read oldest key in keypool failed");
    assert(keypool.vchPubKey.IsValid());
    return keypool.nTime;
}
std::map<CTxDestination, CAmount> CWallet::GetAddressBalances()