Enhancement: Implement hash structure under uploadable-oneblock

[matthewdarwin]

Copy from: streamingfast/node-manager#20

I have reader-node producing lots of one-block files, catching up from block 1.

$ ls reader/work/uploadable-oneblock | wc -l
2601403

It can produce files faster they can be uploaded to s3 bucket.
Of course should also run the merger with the reader in this scenario so less files to upload to s3.