Change snapshot source field into three state

The snapshot field is now boolean that indicates whether the record comes from snapshot or not. We want to change it into three state field that will indicate whether it is from snapshot, last from snapshot or from streaming.
The logic proabbly should be

• snapshot is non-optional field
• the type of the field will be enum with values I ntermediary, L ast, N one
• the default value will be N so it does not need to be included in streaming messages