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.

{
    return std::max(minTxFee.GetFee(nTxBytes), ::minRelayTxFee.GetFee(nTxBytes));
}
CAmount CWallet::GetMinimumFee(unsigned int nTxBytes, unsigned int nConfirmTarget, const CTxMemPool& pool)
{
    
    return GetMinimumFee(nTxBytes, nConfirmTarget, pool, payTxFee.GetFee(nTxBytes));
}
CAmount CWallet::GetMinimumFee(unsigned int nTxBytes, unsigned int nConfirmTarget, const CTxMemPool& pool, CAmount targetFee)
{
    CAmount nFeeNeeded = targetFee;
    
    if (nFeeNeeded == 0) {
        int estimateFoundTarget = nConfirmTarget;
        nFeeNeeded = pool.estimateSmartFee(nConfirmTarget, &estimateFoundTarget).GetFee(nTxBytes);
        
        if (nFeeNeeded == 0)
            nFeeNeeded = fallbackFee.GetFee(nTxBytes);
    }
    
    nFeeNeeded = std::max(nFeeNeeded, GetRequiredFee(nTxBytes));
    
    if (nFeeNeeded > maxTxFee)
        nFeeNeeded = maxTxFee;
    return nFeeNeeded;
}
DBErrors CWallet::LoadWallet(bool& fFirstRunRet)