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