This is a follow up to CSHARP-3162
The getMore test (link) was failing, because getMore command contained incorrect value for batchSize field.
This field is calculated correctly for getMore protocol: AsyncCursor.CalculateGetMoreProtocolNumberToReturn() so this logic could be reused for calculating batchSize value for getMore command as well.
Specification: Behavior of Limit, skip and batchSize and Combining Limit and Batch Size for the Wire Protocol
Also in scope of this ticket json test (getMore.json) should be restored to specification state.