Avro Container File format change: add block size to block descriptor

The new file format in AVRO-160 limits a few use cases that I have found to be important.

A block currently contains a count of the number of records, the block data, and a sync marker.
This change would add the block size, in bytes, along side the number of records.

This allows efficient access to a block's data without the need to decode the data into individual Datums, which is useful for various use cases.