remove/change xor(seed, epoch) in shuffling

[djrtwo]

Issue

get_shuffling xor's the epoch into the seed on the fly. This obfuscates the actual seed being used in shuffle. The reason epoch is in this seed is to enforce a new seed at a reshuffling even in the event that entire epochs of slots have been skipped (i.e. in the unlikely event that randao mix stayed the same and active index root stayed the same).

Proposed implementation

add epoch into the hash calculation in generate_seed.