Rewrite get-file to exit faster when not enough blocks are available

Description

This MR allows to finish get-file with an error as soon as we notice that all peers have answered with their list of blocks but not enough blocks overall are available to make the file.

It also solves a bug where all peers have returned the block data they have but due to an un-droped sender we were still waiting for more answers (which would never arrive)

Closes #6