EIP-3238: Difficulty Bomb Delay to Summer 2022

[q9f]

Delays the difficulty bomb so 30 second blocks won't happen until around summer 2022.

PR: #3238

[q9f]

:O

[tjayrush]

I'm tracking it here: https://ethresear.ch/t/blocks-per-week-as-an-indicator-of-the-difficulty-bomb/12120/17?u=quickblocks. I post an update every week.