Add expected number of blocks for the slot to the Blob struct

[carllin]

Problem

Each blob should contain the number of blocks expected for the slot that contains that blob. This is used to chain together slots in db_ledger.

Proposed Solution

1. Add num_blocks to Blob struct
2. Consume num_blocks in chaining logic in db_ledger insert_data_blobs(), right now num_blocks is assumed to be 1.