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.

{
    set<uint256> result;
    if (pwallet != NULL)
    {
        uint256 myHash = GetHash();
        result = pwallet->GetConflicts(myHash);
        result.erase(myHash);
    }
    return result;
}
CAmount CWalletTx::GetDebit(const isminefilter& filter) const