parallel validator key generation

[skylenet]

Hey @protolambda . Nice tool! I'm using it to generate the genesis.ssz file for some private testnets on the fly. I was using it with 100k+ validators and it was taking me like 10min just to get the files generated. This change makes spawning my networks from scratch quite faster :)

Parallelize the validator creation.

$ time ./eth2-testnet-genesis phase0 --config config.yaml --mnemonics mnemonics.yaml --timestamp 1623884847 --tranches-dir tranches_new --state-output genesis_new.ssz
# 210.45s user 1.42s system 723% cpu 29.301 total

$ time ./eth2-testnet-genesis-old phase0 --config config.yaml --mnemonics mnemonics.yaml --timestamp 1623884847 --tranches-dir tranches_old --state-output genesis_old.ssz
# 121.11s user 3.50s system 120% cpu 1:43.09 total

Time sample from my local dev machine:
20.000 validators from 1 mnemonic was taking 1min43sec. With the change it's taking me 29sec

A quick sanity check:

# Comparing outputs to make sure we didn't break anything
$ diff tranches_new/tranche_0000.txt tranches_old/tranche_0000.txt # returns nothing, so we're good

$ zcli pretty phase0 BeaconState genesis_old.ssz > genesis_old.json
$ zcli pretty phase0 BeaconState genesis_new.ssz > genesis_new.json
$ diff genesis_old.json genesis_new.json # returns nothing, so we're good.