We need to have a possibility to arbitrairly define start point for POST initialisation

[pigmej]

When let's say one is renting GPUs for initialisation or NOT using storage on the same device then it's very inconvenient to hope that the process never restarts.

We need a way to specify --startFrom xyz where xyz is file,labels

[fasmat]

I suggest to add the CLI flags --from and --to and make them the file indices defaulting to 0 and (numUnits * labelsPerUnit * bytesPerLabel) / maxFileSize

Programatically the initializer should just skip files outside of that range during initialization.

[fasmat]

This was added to postcli with https://github.com/spacemeshos/post/pull/179