BDX: Add BlockCount field to BlockData

[holbrookt]

Problem

TransferSession emits BlockData structs when it receives a Block message from a BDX exchange. This BlockData struct does not currently include the BlockCount field, since only Synchronous BDX is implemented right now, so Blocks will always come one at a time.

Once Asynchronous BDX is implemented, BlockData will need to also contain the BlockCount field, since it is not guaranteed that messages will come in order. The application will need to order them using BlockCount.

Proposed Solution

• add BlockCount to BlockData struct
• make sure BlockCount is filled in when Block messages are received, and also modify existing unit tests