Compute pending worker shards on-the-fly

[philipp-fischer]

Currently, energon pre-computes all shard ranges that a worker will walk through. This list can be very large and consume memory unnecessarily.
By defining a function for compute the Nth shard for a worker on-the-fly, this precomputation becomes obsolete.

When the state is saved to disk, only the current index (number of shard ranges already processed) needs to be stored, and potentially a shuffling seed.