scan if in memory is too much

[AnatolyBuga]

It's possible clients will have too much data to hold it in memory all the time. For such usecases we can try to provide an option for lazy scans, leveraging on with_streaming.

Pros:

1. No in memory frame

Cons:

1. Performance will be impacted significantly because reading, assigning weights, has to happen per request.

[AnatolyBuga]

Step 1: Make execute funtion lazy from start to finish
c4dbed6

[AnatolyBuga]

When streaming atm UI wouldn't allow to override "prepared" columns such as risk weight. Perhaps add an overridable method to the DataSet trait which would return Vec of added overridable columns.