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 (pwallet->mapAddressBook.count(r.destination))
            {
                map<CTxDestination, CAddressBookData>::const_iterator mi = pwallet->mapAddressBook.find(r.destination);
                if (mi != pwallet->mapAddressBook.end() && (*mi).second.name == strAccount)
                    nReceived += r.amount;
            }
            else if (strAccount.empty())
            {
                nReceived += r.amount;
            }
        }
    }
}
/**
 * Scan the block chain (starting in pindexStart) for transactions
 * from or to us. If fUpdate is true, found transactions that already
 * exist in the wallet will be updated.
 */
int CWallet::ScanForWalletTransactions(CBlockIndex* pindexStart, bool fUpdate)