fix chunkIdx bug

[cyl19970726]

The current ChunkIdx is calculated as follows in older versions of the code:

• chunkIdx := ds.ChunkIdx() + kvIdx*ds.chunksPerKv + i And the ds.ChunkIdx() code :

  func (ds *DataShard) ChunkIdx() uint64 {
  return ds.shardIdx * ds.chunksPerKv * ds.kvEntries
  }

  Thus , ds.ChunkIdx() means that the starting ChunkIdx of the current data shard.

This caused an error: ds.ChunkIdx() and kvIdx*ds.chunksPerKv have overlapping calculations

In my perspective, the correct calculation formula for current ChunkIdx is as follows :

• chunkIdx := kvIdx*ds.chunksPerKv + i

[blockchaindevsh]

Suggest to rename ChunkIdx() to StartChunkIdx() in order to avoid confusion.