Update chainparams for 0.19

Summary: Update chain params on mainnet. Testnet is still only ~10,000 blocks from the last checkpoint, so skipping updating it for now.

Test Plan: Verify on your own node(s) that this block is valid and hash/chainwork are correct.

Reviewers: deadalnix, Fabien, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2564