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.

{
    if (!CCryptoKeyStore::AddCScript(redeemScript))
        return false;
    if (!fFileBacked)
        return true;
    return CWalletDB(strWalletFile).WriteCScript(Hash160(redeemScript), redeemScript);
}
bool CWallet::LoadCScript(const CScript& redeemScript)
{
    /* A sanity check was added in pull #3843 to avoid adding redeemScripts
     * that never can be redeemed. However, old wallets may still contain
     * these. Do not add them to the wallet and warn. */
    if (redeemScript.size() > MAX_SCRIPT_ELEMENT_SIZE)
    {
        std::string strAddr = CBitcoinAddress(CScriptID(redeemScript)).ToString();
        LogPrintf("%s: Warning: This wallet contains a redeemScript of size %i which exceeds maximum size %i thus can never be redeemed. Do not use address %s.\n",
            __func__, redeemScript.size(), MAX_SCRIPT_ELEMENT_SIZE, strAddr);
        return true;
    }
    return CCryptoKeyStore::AddCScript(redeemScript);
}
bool CWallet::AddWatchOnly(const CScript &dest)