Set the payment amount based on file size instead of fix amount

[floalex]

Currently, we use a fix amount 10 for each transaction when when the user downloads/uploads shards each time, no matter how big or small the piece of file is. For example, owner pays the same amount of lumens for a 5MB piece of data, and also pays the same for a 1KB piece of data :

https://github.com/layr-team/batnode_proto/blob/20f947dea5a25ccbf43e36114979721b61044968/batnode.js#L51

While it works in our alpha phase, in real world situation we will need to calculate the amount for each shard/file based on its size to ensure fair usage.