Compute all committees at the beginning of an epoch, not per-slot

[Arkenan]

Benchmarks have shown that even for 100 elements, calculating each shuffled index individually is about 10 times slower than shuffling the whole list in a single batch. For non-light clients such as ours, this can be a great improvement in block processing times.

[Arkenan]

Completed at #1245 . Total block processing time went from 27 seconds to 19.