Sort transactions by time / block height

[Voxelot]

The current graphql connection for transactions just naively iterates over the main collection of transactions which are ordered by transaction id. Update this collection to return transactions in order by block height / time. This could be done by making a new sorted index of transaction id's by time, or using the existing index for blocks and then flattening the stored transactions ids stored on the sorted blocks.

closes FuelLabs/block-explorer-v2#3