Update defaultAssumeValid and minimumChainWork.

Summary:
Update chain parameters for 0.18.0 release.
Values are based on block height 543000 for main net
and block height 1250000 for testnet.

Test Plan: Build and do Initial block Download

Reviewers: deadalnix, schancel, jasonbcox, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1643