Enhancement: only upload blocks if merger needs them

[matthewdarwin]

Copy from: streamingfast/node-manager#21

I'm only trying to save processing load. If you have an infrastructure that is just booting up (after a crash or whatever), then things are very overloaded as things get in sync uploading tons of one-block files. The more 'unecessary' processing load that can be removed, the better (speaking from an operations point of view).

This is low priority.