Big file synthetic challenges

[rayokamoto]

Right now, synthetic challenges are only done with files that are less than a 1 GiB. This is not reflective of real-world usage which can have many files that multiple GiB or even TiB. As such, we need to incorporate challenges that have larger files (and hence larger pieces to challenge miners on).

The generation of these larger files will be less frequent than the smaller files but the rate at which these smaller and larger pieces will not change.