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);
    
    SetMinVersion(FEATURE_HD);
    
    
    
    CHDChain newHdChain;
    newHdChain.masterKeyID = pubkey.GetID();
    SetHDChain(newHdChain, false);
    return true;
}
bool CWallet::SetHDChain(const CHDChain& chain, bool memonly)
{
    LOCK(cs_wallet);
    if (!memonly && !CWalletDB(strWalletFile).WriteHDChain(chain))
        throw runtime_error(std::string(__func__) + ": writing chain failed");
    hdChain = chain;
    return true;
}
bool CWallet::IsHDEnabled()
{
    return !hdChain.masterKeyID.IsNull();
}
int64_t CWalletTx::GetTxTime() const