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.

{
    CAmount nTotal = 0;
    {
        LOCK2(cs_main, cs_wallet);
        for (map<uint256, CWalletTx>::const_iterator it = mapWallet.begin(); it != mapWallet.end(); ++it)
        {
            const CWalletTx* pcoin = &(*it).second;
            nTotal += pcoin->GetImmatureWatchOnlyCredit();
        }
    }
    return nTotal;
}
void CWallet::AvailableCoins(vector<COutput>& vCoins, bool fOnlyConfirmed, const CCoinControl *coinControl, bool fIncludeZeroValue) const
{
    vCoins.clear();
    {
        LOCK2(cs_main, cs_wallet);
        for (map<uint256, CWalletTx>::const_iterator it = mapWallet.begin(); it != mapWallet.end(); ++it)
        {
            const uint256& wtxid = it->first;
            const CWalletTx* pcoin = &(*it).second;
            if (!CheckFinalTx(*pcoin))
                continue;
            if (fOnlyConfirmed && !pcoin->IsTrusted())
                continue;
            if (pcoin->IsCoinBase() && pcoin->GetBlocksToMaturity() > 0)
                continue;
            int nDepth = pcoin->GetDepthInMainChain();
            if (nDepth < 0)