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.

{
    
    
    
    int nMinOrderPos = std::numeric_limits<int>::max();
    const CWalletTx* copyFrom = NULL;
    for (TxSpends::iterator it = range.first; it != range.second; ++it)
    {
        const uint256& hash = it->second;