add lag / attribution window to incremental

[rudolfix]

Background In many cases certain portion of data should be reacquired during incremental loading. Ie we want to always capture last 7 days of data when getting daily analytics report. Or we want to refresh slack message replies and we have a moving window of 7 days for that. Technically we would always pass start_date +/- the lag to the function accepting incremental

Requirements

1. • [ ] add new optional filed to Incremental class that will hold the lag.
2. • [ ] lag should be expressed as float and will be interpreted depending on the type of the cursor. for datetimes it is a lag value in seconds, for any other type use + / - operator depending on the last_value_func
3. • [ ] we support only min and max for last_value_func. for customs we do not have "+" operator defined.